We would update the set of seen GTIDs as soon as we saw a GTID_EVENT,
which arrives before a transaction. This would mostly work fine, but
if we lost the connection to MySQL in the middle of a large transaction
we would persist that the transaction had been processed as soon as the
transaction had started. When the connection was reestablished, we
would not process the transaction again, which meant that we only
applied parts of it.
Fix this by updating the seen GTIDs at the end of the transaction
instead.
