Set seqnum appropriately for non-default kvstore handle

For a non-default kvstore instance, it is observed that snapshot_open
operation after a db commit would always fail with error code,
FDB_RESULT_NO_DB_INSTANCE. During file open, the db header scanner was
unable to find the correct header since seqnum was set to default kvstore's
seqnum instead of non-default kvstore's seqnum.

Fixed _fdb_open() to set seqnum correctly for non-default kvstore handle.