Add option to ignore thinktime for rated IO

By default, fio will ignore thinktime when calculating the next
time to issue and IO, if rated IO is specified. This leads to
fio entering a catch-up type of mode after doing the specified
sleep. For some workloads, that may not be useful. If someone
asks for a specific amount of IOPS and sets a thinktime, they
may want to exclude the sleep time.

Fixes: https://github.com/axboe/fio/issues/497
Signed-off-by: Jens Axboe <axboe@kernel.dk>