dsdb: Log the transaction duraton.

dsdb: Log the transaction duraton.

This is not a general purpose profiling solution, but these JSON
logs are already being generated and stored, so this is worth adding.

This will allow administrators to identify long running
transactions, and identify potential performance bottlenecks.

This complements a similar patch set to log authentication duration.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Mon Jun 25 11:16:18 CEST 2018 on sn-devel-144