common/qat: support dual threads for enqueue/dequeue

common/qat: support dual threads for enqueue/dequeue

Remove the limitation whereby enqueue and dequeue must be
done in same thread.
The inflight calculation is reworked to be thread-safe for 2
threads - note this is not general multi-thread support, i.e
all enqueues to a qp must still be done in one thread and
all dequeues must be done in one thread, but enqueues and
dequeues may be in separate threads.
Documentation updated.

Signed-off-by: Fiona Trahe <fiona.trahe@intel.com>
Signed-off-by: Arek Kusztal <arkadiuszx.kusztal@intel.com>