Revert "MEDIUM: queue: determine in process_srv_queue() if the proxy is usable"

Revert "MEDIUM: queue: determine in process_srv_queue() if the proxy is usable"

This reverts commit de814dd4228fa5e528d9ac1f0e1c4c7325ee52d3.

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.