[PATCH] add --throttle option to pgbench

Hello,

Please find attached a small patch to add a throttling capability to 
pgbench, that is pgbench aims at a given client transaction rate instead 
of maximizing the load. The throttling relies on Poisson-distributed 
delays inserted after each transaction.

I wanted that to test the impact of various load levels, and for 
functionnal tests on my laptop which should not drain the battery.
 sh> ./pgbench -T 10 -c 2 --throttle 10tps test starting vacuum...end. transaction type: TPC-B (sort of) scaling
factor:1 query mode: simple number of clients: 2 number of threads: 1 duration: 10 s number of transactions actually
processed:214 tps = 21.054216 (including connections establishing) tps = 21.071253 (excluding connections
establishing)

-- 
Fabien.