libctdb: always check header hasn't changed on local tdb

libctdb: always check header hasn't changed on local tdb

The code on which this is based could alter the header: a normal client
can't.  If we use this differently later we can change this.  For the
moment it's a nice extra check.

We optimize out the record write altogether when the record hasn't
changed, rather than just suppressing the seqnum update.

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