[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.