dsdb: Remove 120 second delay and USN from schema refresh check

dsdb: Remove 120 second delay and USN from schema refresh check

We now refresh it once the schema changes, so that replication can
proceed right away.  We use the sequence number in the metadata.tdb.

The previous commit added a cache for this value, protected by
tdb_seqnum().

metadata.tdb is now opened at startup to provide this support.

Note that while still supported, schemaUpdateNow is essentially rudundent:
instead, to ensure we increment the sequence number correctly, we unify that check
into repl_meta_data at the transaction close.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>