Leave sequence number alone when merely migrating records.

Leave sequence number alone when merely migrating records.

(Based on earlier version from Ronnie which modified tdb; this one
is standalone).

When storing records in a tdb that has "automatic seqnum updates"
also check if the actual data for the record has changed or not.

If it has not changed at all, except for possibly the header,
this is likely just a dmaster migration operation in which case
we want to write the record to the tdb but we do not want the tdb
sequence number to be increased.

This resolves the problem of notify.tdb being thrashed under load:
the heuristic in smbd to only reread this when the sequence number
increases (rarely) breaks down.

Before, running nbench --num-progs=512 across 4 nodes, we saw numbers like:
 512      1496  118.33 MB/sec  execute 60 sec  latency 0.00 msec
And turning on latency tracking, this was typical in the logs:
 ctdbd: High latency 9380914.000000s for operation lockwait on database notify.tdb

After this commit:
  512      2451  143.85 MB/sec  execute 60 sec  latency 0.00 msec
And no more latency messages...

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>