Limited the number of pending operations per request

Limited the number of pending operations per request

Each pending asynchronous operation holds a reference in
r->main->blocked, an 8-bit field, and the increments were not checked.
With 256 operations pending on a single request the counter wrapped to
zero, after which the request could be freed while those operations
still referenced its pool.  This is the same overflow that 4ec67cf86
addressed for r->main->count, and left unaddressed for r->main->blocked.

Operations beyond the limit are now performed synchronously where a
synchronous path exists, and refused otherwise, so that the counter
cannot overflow.

Reported by cyberlancer.