Simplify Travis-CI file and extend inline comments.

Upgrade to using Ubuntu 20.04 (Focal) as the baseline distro version
now that Travis-CI has made it available. Drop Xenial and all the
excess repositories Xenial needed. Now we only Focal and one Bionic
build to keep things simple and streamlined.

Keep GCC-7/Clang-7 as the older compiler, and start using GCC-10
and Clang-10 as the newer compiler. Assume that if both of them
build OK, than the intermediate versions would be OK as well.

Print 'apt-cache policy' to make it transparent in build logs what
repositories was used for build dependencies.

Remove temporary workaround from homebrew install step as Travis-CI has
fixed the original issue.

Revert ignoring results form build that previously failed on the test
main.thread_pool_info as MDEV-20372 is not fixed.

Keep arm64 failures ignored due to MDEV-23955.

Allow failures for the test main.column_compression 'innodb' due
to MDEV-23954.

innodb_preshutdown(): On innodb_fast_shutdown=0, only wait for
transactions to exit if the transaction system had been initialized.

Reviewed by: Vladislav Vaintroub

In MDEV-15053 (commit b1ab211d)
we inadvertently removed a check whether innodb_flush_neighbors is 0,
and thus started treating only the value 1 in a special way.

buf_flush_check_neighbors(): Add the parameter contiguous,
which can be set to skip the check for non-contiguous page number ranges.

Reviewed by: Thirunarayanan Balathandayuthapani

This was missed in commit 5155a300.

This was missed not only in
commit 5e62b6a5 but also in
commit a9550c47.

test_sql_service plugin added and employed in test_sql_service.test.

Remove stale references to srv_sys.sys_threads and srv_sys.mutex
that were removed in
commit 5e62b6a5.

We can simply use C++11 std::atomic for avoiding undefined behaviour
related to concurrent stores to a shared variable. On most if not all
ISAs, std::memory_order_relaxed loads and stores will not really
differ from non-atomic loads or stores.

In fts_optimize_remove_table(), InnoDB tries to access the
fts_optimize_wq after shutting down the fts optimize thread.
This issue caused by the commit a41d4297.
Fix should check for fts optimize thread shutdown state
before checking fts_optimize_wq.

LATCH_ID_OS_AIO_READ_MUTEX,
LATCH_ID_OS_AIO_WRITE_MUTEX,
LATCH_ID_OS_AIO_LOG_MUTEX,
LATCH_ID_OS_AIO_IBUF_MUTEX,
LATCH_ID_OS_AIO_SYNC_MUTEX: Remove. The tpool is not instrumented.

lock_set_timeout_event(): Remove.

srv_sys_mutex_key, srv_sys_t::mutex, SYNC_THREADS: Remove.

srv_slot_t::suspended: Remove. We only ever assigned this data member
true, so it is redundant.

ib_wqueue_wait(), ib_wqueue_timedwait(): Remove.

os_thread_join(): Remove.

os_thread_create(), os_thread_exit(): Remove redundant parameters.

These were missed in commit 5e62b6a5.

This was missed in commit 30ea63b7.

This has been unused from the very beginning
(mysql/mysql-server@d5e512ae7e37cd1f70c44a3f12205d70b13118ab).

MySQL.com and Percona packages can be root auth_socket only. MariaDB uses unix_socket.
As the root user, as the default Debian maintaince user, needs to
be accessible to run mariadb-upgrade for a start, we make it accessible
again.

ibuf_merge_or_delete_for_page(): Do not attempt to invoke
ibuf_delete_recs() on a page of the change buffer itself.
The caller could already be holding ibuf->index->lock,
and an attempt to acquire it in S mode would hang the release server
or cause an assertion failure in rw_lock_s_lock_func() in a debug
server.

This problem was reproducible on 1 out of 2 runs of the following:
./mtr --no-reorder \
innodb.innodb-page_compression_default \
innodb.innodb-page_compression_snappy \
innodb.innodb-page_compression_zip \
innodb.innodb_wl6326_big innodb.xa_recovery

This was missed in commit 2c252ba9
(MySQL 5.5.42, MariaDB 5.5.42).

This bug happened when the HEAP temporary table used for the derived table
created for a derived handler of a remote engine of the federated type
became full and was converted to an Area table. For this conversion
the tmp_table_param parameter must be always taken from the select_unit
object created for the result of the derived table.

Fix typo.