Revert "MINOR: queue: use atomic-ops to update the queue's index"

Revert "MINOR: queue: use atomic-ops to update the queue's index"

This reverts commit 1335eb9867b76c8e4570ad4242dc728287af3d56.

The recent changes since 5304669e1 MEDIUM: queue: make
pendconn_process_next_strm() only return the pendconn opened a tiny race
condition between stream_free() and process_srv_queue(), as the pendconn
is accessed outside of the lock, possibly while it's being freed. A
different approach is required.