samba-tool: Rework transations/locks to hold a lock during mdb backup

samba-tool: Rework transations/locks to hold a lock during mdb backup

We now also get sidForRestore under that lock, rather than
after the backup.

This avoids using the database again after the backup process

While not entirely clear how/why this matters with LMDB
as seen in Fedora 34, likely due to the same issues
seen with 0.9.26 or later fixed by commmit
bb3dcd403ced922574a89011dd3814c4fe87dd76.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14676

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>