s3:registry: fix seqnum race in regdb_fetch_keys_internal

s3:registry: fix seqnum race in regdb_fetch_keys_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 66fcac5e479a530091ecb43d9f8cf90f4351ad17)