s3:registry: fix seqnum race in fetch_values_internal

s3:registry: fix seqnum race in fetch_values_internal

This prevents race between fetching seqnum and key content.

Because there is currently no way to atomically fetch the
record along with the seqnum, I use a loop.
This is far from optimal and should should ideally be done
differently. But for now it fixes the race.
(cherry picked from commit 13bccba3c2f6e6fdda2b4a40dd4b1e250a98a7ef)

Conflicts:

	source3/registry/reg_backend_db.c