partition: correcting lock ordering

partition: correcting lock ordering

A schema reading bug was traced to a lock ordering issue in partition.c.
This patch fixes the problem by:
1. Releasing locks/transactions in the order they were acquired.
2. Always lock/start_trans on metadata.tdb first, before any other
databases, and release it last, after all others. This is so that we are
never exposed to MDB's lock semantics, which we don't support.

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