Revert "MINOR: queue: update proxy->served once out of the loop"

Revert "MINOR: queue: update proxy->served once out of the loop"

This reverts commit 3e92a31783b545dd58c4be6c588808763e0042bc.

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.