MDEV-6462: Slave replicating using GTID doesn't recover correctly when master crashes in the middle of transaction

If the slave gets a reconnect in the middle of a GTID event group, normally
it will re-fetch that event group, skipping the first part that was already
queued for the SQL thread.

However, if the master crashed while writing the event group, the group is
incomplete. This patch detects this case and makes sure that the
transaction is rolled back and nothing is skipped from any following
event groups.

Similarly, a network proxy might cause the reconnect to end up on a
different master server. Detect this by noticing a different server_id,
and similarly in this case roll back the partially received group.