Fix synchronisation

[lgrahl]

This library is not thread-safe and thus does not need synchronisation. All synchronized blocks and synchronised lists etc. should be removed/replaced.

Since the WebSocket implementation does use threads internally, we still need synchronisation between it and the public API surface. I see a couple of options:

1. Make everything synchronized.
2. Introduce an API to hand in a lock from the application that is being used for pretty much everything.
3. Get rid of the WebSocket threads and allow this library to be run single-threaded (my preference, mid-term).
4. Remove the WebSocket transport. Create a transport interface and let the application create its own WebSocket transport (my preference, long-term).

[dbrgn]

I'd explore 3, potentially using a single thread executor service: https://github.com/TakahikoKawasaki/nv-websocket-client#connect-to-server-asynchronously

[ovalseven8]

I'm not entirely sure what you mean, @dbrgn. Can you explain how you would structure that?

[ovalseven8]

For the time being #125 basically introduces @lgrahl's 1./2.