s3:dbwrap_ctdb: exit early when nothing has been written in transaction_commit.

s3:dbwrap_ctdb: exit early when nothing has been written in transaction_commit.

This skips update of the __db_sequence_number__ record when nothing else has
been written. There are transactions that are just openend and then nothing
is written until transaction_commit is called. This is for instance the case
with registry initialization routines: They start a transaction and only
write somthing when the registry has not been initialized yet.
So this change will skip many db_seqnum bumps and TRANS3_COMMIT roundtrips.

Michael