It's possible to establish Galera multi-cluster setups connected
through the native replication when every Galera cluster is configured
to have a separate domain ID.
For this setup to work, we need to replace domain ID values in generated
GTID events when they are written at transaction commit to the values
configured by Wsrep replication.

At the same time, it's possible that the GTID event already contains
a correct domain ID if it comes through the native replication from
another Galera cluster.
In this case, when such an event is applied either through a native
replication slave thread or through Wsrep applier, we write GTID event
on transaction start and avoid writing it during transaction commit.

The code contained multiple problems that were fixed:
- applying GTID events didn't work because it's applied without a
running server transaction and Wsrep transaction was not started
- GTID event generation on transaction start didn't contain proper
"standalone" and "is_transactional" flags that the original applied
GTID event contained
- condition determining that GTID event is written on transaction start
to avoid writing it on commit relied on the fact that the GTID event
is the first found in transaction/statement caches, which wasn't the
case and resulted in duplicate GTID events written
- instead of relying on the caches to find a GTID event, a simple check
is introduced that follows the exact rules for checking if event is
written at transaction start as described above
- the test case is improved to check that exact GTID events are
applied after two Galera clusters have synced.
Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>