This patch fixes a problem that arises when a Galera node acts as a
replica for native replication. When parallel applying is enabled, it
is possible to end up with attempts to write binlog events with gtids
out of order. This happens because when multiple events are delivered
from the native replication stream and applied in concurrently, it is
for them to be replicated to the Galera cluster in an order which is
different from the original order in which they were committed in the
aync replication master.
To correct this behavior we now wait_for_prior_commit() before
replicating changes though galera. As a consequence, parallel appliers
may apply events in parallel until the galera replication step, which
is now serialized.