[PATCH] add --throttle to pgbench (submission 3)

Add --throttle to pgbench

Each client is throttled to the specified rate, which can be expressed in 
tps or in time (s, ms, us). Throttling is achieved by scheduling 
transactions along a Poisson-distribution.

This is an update of the previous proposal which fix a typo in the sgml 
documentation.

The use case of the option is to be able to generate a continuous gentle 
load for functional tests, eg in a practice session with students or for 
testing features on a laptop.

-- 
Fabien.