Fix thread safety reentry

[phodge]

Fixes broken tests described in #201 by switching to an RLock. Also added a bunch of comments to explain what's going on.

[phodge]

@jamielennox: Unfortunately I haven't been able to find a better solution than RLock, but have added some extra comments to try and make life easier for whoever has to maintain this in the future. I added a timeout of 10 seconds for the thread locking to try and prevent tests blocking indefinitely, which might be a small quality of life improvement but it ended up being a bit verbose trying to support python2. If you think it's too much I can rip it all out and go back to just with _send_lock:.

[phodge]

Hey @jamielennox, just wanted to check in again and see if there's anything else I can do to help get this merged and released?

[jamielennox]

I'm sorry this has been so long. Admittedly i'm a little scared of this one, i've no idea what edge cases it might trigger but we can release it and see.