Implement request queue overflow protection.

Before this change in case of request queue overflow driver just froze the
device queue for 100ms to retry after. It was pretty bad for performance.
This change introduces SIM queue freezing when free space on the request
queue drops below 255 entries (worst case of maximum I/O size S/G list),
checking for a chance to release it on I/O completion. If the queue still
get overflowed somehow, the old mechanism is still in place, just with
delay reduced to 10ms.

With the earlier queue length increase overflows should not happen often,
but it is still easily reachable on synthetic tests.