Internal checks to detect client streams/circuits whose sock request vanished

Tor has a cycle for handling streams, where it attaches the stream to a circuit, gives up after a while, attaches it to another, etc.

Tor also has a timeout after which it hangs up on the socks connection, and some logic to notice that a socks connection has hung up so it should cancel streams.

Long ago, it was quite common for me to see the "keep trying" cycle continue even after the socks request had vanished. Those were bugs.

I haven't seen them so much recently, but it would be smart to add some logic to check the consistency of "for each thing we're trying to handle, is there still a connection around that's asking us to handle it." If we had that logic, it might help catch the root cause of #2983 (moved).