Factor WaitQueue out into a separate file

[mstewartgallus]

I'm a bit disappointed that I couldn't figure out how to factor out more of the code implementing extra::sync but I feel this is an okay start. Also I added some documentation explaining that WaitQueue isn't thread safe, and needs an exclusive lock.

@bblum

[bblum]

As I commented before, I don't see a reason the WaitQueue should be in its own file. There is nothing besides the rest of the stuff in sync that needs to use it.

Please poke me on IRC so we can discuss how to organize these modules. I think we have drastically different views of what the final version should end up looking like.

[mstewartgallus]

So, me, and bblum had a long chat on IRC about this. We still have our disagreements about when modules should be split up into multiple files, and how encapsulated classes should be but I've compromised on a much less drastic change that would allow a possible split up if needed but does not split up sync.rs yet.

@bblum So I've changed the pull request.