if ssl_cipher is not specified, it must be NULL, not ""

MDEV-18122 Assertion 'table->versioned() == m_prebuilt->table->versioned()' failed in ha_innobase::open

Closes #1134

and gitignore myrocks_hotbackup (as it's now generated)

Closes #1081

Closes #1080

It's too late. Might work or not (and on buster it doesn't).

MDEV-18057 Assertion `(node->state == 5) || (node->state == 6)' failed in row_upd_sec_step upon DELETE after UPDATE failed due to FK violation

The idea of the fix: reset state from previous query.

row_upd_clust_step(): reset cached index before updating a clustered index

Closes #1133

Fix for galera_3nodes galera_ipv6_mysqldump and galera_ipv6_mariabackup

Remove fil_node_t::sync_event.

I had a discussion with kernel fellows and they said it's safe to call
fsync() simultaneously at least on VFS and ext4. So initially I wanted
to disable check for recent Linux but than I realized code is buggy.

Consider a case when one thread is inside fsync() and two others are
waiting inside os_event. First thread after fsync() calls os_event_set()
which is a broadcast! So two waiting threads will awake and may call
fsync() at the same time.

One fix is to add a notify_one() functionality to os_event but I decided
to remove incorrect check completely. Note, it works for one waiting
thread but not for more than one.

IMO it's ok to avoid existing bugs but there is not too much sense in
avoiding possible(!) bugs as this code does.

fil_space_t::is_in_rotation_list(), fil_space_t::is_in_unflushed_spaces():
Replace redundant bool fields with member functions.

fil_node_t::needs_flush: Replaces fil_node_t::modification_counter and
fil_node_t::flush_counter. We need to know whether there _are_ some
unflushed writes and we do not need to know _how many_ writes.

fil_system_t::modification_counter: Remove as not needed.
Even if we needed fil_node_t::modification_counter, every file
could have its own counter that would be incremented on each write.

fil_system_t::modification_counter is a global modification counter
for all files. It was incremented on every write. But whether some
file was flushed or not is an internal fil_node_t deal/state and
this makes fil_system_t::modification_counter useless.

Closes #1061

MDEV-18379: Unification of check for IPv6

Currently, the three-node mtr suite for Galera (galera_3nodes)
uses a separate IPv6 availability check using the "have_ipv6.inc"
file. This check duplicates a more accurate check at suite.pm
level, which can be used by including the file "check_ipv6.inc".

This patch removes this discrepancy between suites. In addition,
one of the files in the galera_3nodes suite does not contain the
option "--bind-address=::" which is needed for the test to work
correctly with IPv6 (at least on some systems), since without
it the server will not wait for connections on the IPv6 interface.

https://jira.mariadb.org/browse/MDEV-18379

Add a simplest regression test. Specifically, I want to be sure that
SYS_COLUMNS.LEN is increased.

Closes #1123

WSREP_LIB_MAINTAINER_MODE was introduced in wsrep-lib cmake options
to enable -Werror. The maintainer mode is disabled by default in order
not to fail compilation on every compiler warning. The wsrep-lib
maintainer mode can be enabled via -DWSREP_LIB_MAINTAINER_MODE:BOOL=ON.

* Made galera_3nodes.galera_ipv6_mariabackup deterministic with respect
  to donor selection, fixed assert grep definitions to match
  Galera 4 behavior and recorded.
* Removed extra connection setup from galera_3nodes.galera_ipv6_mysqldump
  and recorded.

fix of incorrect 10.3 merge

Call to wsrep_handle_SR_rollback was missing check that wsrep_on is true.

MDEV-18372: Minor MDEV-17973-related merge issue to 10.3

Restore changes from 10d347dc that were lost in a merge

Use the same data type 'ulong' to avoid type mismatch on Windows
and on 32-bit systems.

FIXME: The correct data type should probably be 64-bit.

When innobase_allocate_row_for_vcol() returns true (for failure),
it may already have invoked mem_heap_create(). However, some callers
would fail to invoke mem_heap_free().