Run I/O on separate thread

[wch]

Closes #56. This PR runs the websocket I/O on a separate thread, instead of polling using later(). When a WebSocket exists but is idle, in a terminal it previously consumed about 2.5% CPU on my computer, and in RStudio it previously consumed 0.7%. Now it consumes 0% because the I/O thread's event loop works without using polling (I believe it uses something like epoll or select under the hood).

Also, with Chromote, it previously idled at ~6.5% CPU, but after this change and rstudio/chromote#16, it idles at 0%.

TODO:

• Document that if even you lose the reference to an open WebSocket object, it will continue to exist until the websocket is closed; after that point, a gc() will GC it.
• Update existing tests for new non-polling behavior.
• Add test that if ref to ws is gone, and the websocket is closed from the other end, that a gc() will cause the finalizer to run.
• Add test for running in private loop.
• Add test that a failed connection gets finalized, even without a close()
• Add substitutes for Rcerr and Rcout that are safe to run on background thread.

[jcheng5]

Feel free to remove wsRestart and wsReset if you feel like it