Avoid memory leak with weakref, avoid race condition, log closures

[BenjaminPelletier]

This suggestion implements my comments on 1408 plus a few additional things:

• Avoids memory leak by using weakref in periodic closure check
• Avoids race condition between closure and new request by adding _closure_lock
• Informs user when automatic closure occurs via log message, including differentiating UTMClientSession instances with int ID
• Increases keepalive period to just under max keepalive time likely used by external infrastructure

Tested in current state and verified a large number of sessions (90+) and closures. Tested cherrypicking 1411 and verified a smaller number of sessions and closures.