Draft: ZeroMQ socket synchronization, spawning_complete and several cosmetic fixes

[sergii-volokh]

This PR fixes:

• zeromq socket synchronization: the socket is not thread safe, but two threads (Runner#Sender and Runner#Receiver) may send messages concurrently; this bug led to silent zmq connection termination and then heartbeats timeout;
• 'spawning_complete' message is not "valid 'spawn' message";
• locust master expects 'user_count' instead of 'count' in 'spawning_complete';
• add current memory usage stats to heartbeats;
• a bit of cosmetics on threads names.

[sergii-volokh]

For the moment: connection hangup still reproducible... looking for root cause...