This is follow-up patch. Avoid calling constructor/destructor twice.

This is a follow-up to commit 9581c4a8
which added a memcpy() call to rtr_copy_buf(), to copy
rw_lock_t debug_latch. This could emit a warning.

A cleaner approach is to make buf_block_t::debug_latch a pointer,
so that we can avoid copying it. An even cleaner approach would be to
redesign the InnoDB SPATIAL INDEX code so that the function
rtr_copy_buf() is not needed at all.

avoid calling SSL_get_verify_mode() and  SSL_get_verify_depth(),
since yassl implementation of the function accesses the acceptor context,
which can be freed on FLUSH SSL command.

purge_sys_t::m_enabled transition to std::atomic.

enabled_latched() doesn't make much sense: in this particular case it is
as fast as atomic load. The sole caller has to reload it's value anyway,
due to rw_lock_x_lock(&purge_sys.latch) issuing acquire memory barrier.

When purge_sys_t::close() is reached, m_enabled must be false, otherwise
we may free members, which are still in use by the coordinator thread.

purge_sys_t::m_paused transition to Atomic_counter.

paused_latched() doesn't make much sense: in this particular case it is
as fast as atomic load. The sole caller has to reload it's value anyway,
due to rw_lock_x_lock(&purge_sys.latch) issuing acquire memory barrier.

onlineddl_rowlog_rows transition to Atomic_counter.

os_aio_validate_count transition to Atomic_counter.

ibuf_t::n_merges transition to Atomic_counter.

dict_table_t::n_foreign_key_checks_running transition to
Atomic_counter.

trx_sys_t::rseg_history_len transition to Atomic_counter.

fil_space_t::redo_skipped_count transition to Atomic_counter.

srv_conc_t.n_active and srv_conc_t.n_waiting transition to
Atomic_counter.

btr_defragment_compression_failures, btr_defragment_failures and
btr_defragment_count transition to Atomic_counter.

os_thread_count transition to Atomic_counter.

Almost trivial rw_lock_t::waiters transition. Since C++11 doesn't
seem to allow mixed (atomic and non-atomic) access to atomic variables,
we have to perform atomic initialisation.

Almost trivial rw_lock_t::lock_word transition. Since C++11 doesn't
seem to allow mixed (atomic and non-atomic) access to atomic variables,
we have to perform atomic initialisation.

Also made previously broken code in gis0sea.cc even more broken. It is
unclear how it was supposed to work and what exactly it was supposed to
do.

os_total_large_mem_allocated transition to Atomic_counter.

Replaced simple_atomic_counter with Atomic_counter.

Trivial buf_tmp_buffer_t::reserved transition.

Relaxed memory order looks suspicious when used by methods that have
acquire()/release() names. Especially if intention is to transfer some
variable or memory region from one thread to another. Or is memory
ordering guaranteed by something else, e.g. some mutex that protects
broader range of acquire/release functionality?

Almost trivial TTASEventMutex::m_lock_word transition. Since C++11 doesn't
seem to allow mixed (atomic and non-atomic) access to atomic variables,
we have to perform all accesses atomically.

Almost trivial TTASMutex::m_lock_word transition. Since C++11 doesn't
seem to allow mixed (atomic and non-atomic) access to atomic variables,
we have to perform all accesses atomically.

dict_table_t::n_ref_count transition to Atomic_counter.

Trivial srv_running transition.

trx_t::n_ref transition to Atomic_counter.

rw_trx_hash_element_t::no transition to Atomic_counter. Since C++11
doesn't seem to allow mixed (atomic and non-atomic) access to atomic
variables, we have to perform atomic initialisation.

Almost trivial trx_sys_t::m_rw_trx_hash_version transition. Since C++11
doesn't seem to allow mixed (atomic and non-atomic) access to atomic
variables, we have to perform atomic initialisation.

trx_sys_t::m_max_trx_id transition to Atomic_counter. Since C++11 doesn't
seem to allow mixed (atomic and non-atomic) access to atomic variables,
we have to perform atomic initialisation.

fil_validate_count transition to Atomic_counter.

Added Atomic_counter class to replace big set of atomic operations uses
in InnoDB, as well as in the server.

The fix for MDEV-17901 did not cover cases where the AUTO_INCREMENT
column was not dropped, but some other columns before it were.

commit_cache_norebuild(): Revert the MDEV-17901 fix.

dict_index_t::clear_instant_alter(): Update table->persistent_autoinc.

innobase_instant_try(): Only try to update the hidden metadata
record if the number of columns is changing (increasing) or
a metadata BLOB is being added due to permuting or dropping columns
for the first time.

dict_table_t::instant_column(), ha_innobase_inplace_ctx::instant_column():
Return whether the metadata record needs to be updated.

MDEV-17959 Assertion `opt_bootstrap || mysql_parse_status || thd->lex->select_stack_top == 0' failed in parse_sql upon DELETE HISTORY under ORACLE mode

Ubuntu 14.04 Trusty is LTS until April 2019 and since we are
approaching that, upgrade to Xenial which is LTS until April 2021.

One bonus of this upgrade is that the Travis Xenial virtual images
have a much bigger disk size, thus no need to limit ccache below
the default of 5G anymore.

Also had to remove linux/clang-8 toolchain testing since
llvm-toolchain-xenial is not listed at:
https://github.com/travis-ci/apt-source-safelist/blob/master/ubuntu.json

And fails with:
Disallowing sources: llvm-toolchain-xenial
...
E: Unable to locate package clang-8
E: Unable to locate package llvm-8-dev

10.4 travis - newer compilers, ignore common clang warnings

MDEV-18072 Assertion `is_null() == item->null_value || conv' failed in Timestamp_or_zero_datetime_native_null::Timestamp_or_zero_datetime_native_null upon query with GROUP BY

MDEV-18070 Assertion `nanoseconds <= 1000000000' failed in Temporal::add_nanoseconds_ssff with TIME_ROUND_FRACTIONAL

The previous patch was erroneously marked as MDEV-18072.
It was actualy for MDEV-18070.

MDEV-18072 Assertion `is_null() == item->null_value || conv' failed in Timestamp_or_zero_datetime_native_null::Timestamp_or_zero_datetime_native_null upon query with GROUP BY

This assertion should have been relaxed when implementing the first part of
MDEV-15563: instant removal of NOT NULL attribute for ROW_FORMAT=REDUNDANT
tables.

For ROW_FORMAT=REDUNDANT, there is no bitmap of null columns;
the null flags are encoded in the end offset of each field.
We do not really care about the number of fields that can be NULL.