pgbench throttling latency limit

Add --limit to limit latency under throttling

Under throttling, transactions are scheduled for execution at certain
times. Transactions may be far behind schedule and the system may catch up
with the load later. This option allows to change this behavior by
skipping transactions which are too far behind schedule, and count those
as skipped.

The idea is to help simulate a latency-constrained environment such as a
database used by a web server.

--
Fabien.