dsdb: The schema should be reloaded during the transaction

dsdb: The schema should be reloaded during the transaction

Reload the schema just after getting the tranaction lock
but before the transaction counter is bumped.

This ensures we reload the schema exactly once but with
the DB locked.

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