Clean websocket Server Shutdown

[jkuszmaul]

Currently, when shutting down, the websocket server will sometimes end up corrupting/overwriting random bits of memory and cause seg faults while the process is shutting down (this is mostly an issue when running tests while also having the UI open). I do not believe that there is currently a clean way to shut down the websocket server (there may or may not be any mechanism for it in the websocket library).

Reproduce by:

1. Build //control:control_test locally.
2. Open at least 3 tabs of UI pages (not sure why, but for some reason it always worked fine with just 2 windows/tabs on my machine).
3. Run //control:control_test
4. Generally speaking, it will then segfault (sometimes SIG_ABRT gets thrown, noting some corrupted doubly-linked list) in between Square and StartInIrons tests (may require running once or twice; I never needed more than 3 tries to reproduce the segfault, and rarely even a second).
5. If you redo this, but with the hub_ member variable in //ui:server removed (i.e., clearing out the websocket server), then there will be no issues.