s3:registry: update the seqnum in the subkey cache at the end of regval_store_keys

s3:registry: update the seqnum in the subkey cache at the end of regval_store_keys

The purpose is to prevent next reads from going to disk.

Note that this will currently only be effective with local tdbs, not
with ctdb: For tdb, store and delete bump the seqnum while transaction
commit does not. For ctdb, transaction commit bumps the seqnum, while
store and delete don't... This needs fixing (in ctdb).
(cherry picked from commit 16d83149c1b5620598edd37bbd1a73bebec82b6e)