Fix thread hang when using async engines (libaio,etc.) when too low of a iops rate...

Fix thread hang when using async engines (libaio,etc.) when too low of a iops rate is specified.

Rate limiting logic was using thread_data->cur_depth to decide the
min_evts number to ask for during its "flush" prior to sleeping.
td->cur_depth, however, does not properly track in-flight IOs submitted
to the async engines.  Added field to thread_data structure and use
that, instead, to track IOs currently in flight.

Signed-off-by: Ryan Marchand <rmarchan@amazon.com>
Signed-off-by: Steven Noonan <snoonan@amazon.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>