Replace netmgr quantum with loop-preventing barrier

Instead of using fixed quantum, this commit adds atomic counter for number of items on each queue and uses the number of netievents scheduled to run as the limit of maximum number of netievents for a single process_queue() run.

This prevents the endless loops when the netievent would schedule more netievents onto the same loop, but we don't have to pick "magic" number for the quantum.

(cherry picked from commit 9e3cb396)