Changes

A couple of stability improvements.

Replace some throws with logs. The dash core refactor aggressively made some old catch clauses re-throw instead of log. Turns out that some of these were hit in the wild! In particular, there are edge cases around the socket lifecycle where dash may attempt to send messages on a socket that's already closed. The old behavior has been restored.
The new Mutex container helps ensure access to certain variables is synchronized. It doesn't totally foreclose such bugs, but it makes them harder to write. (And this is as good an opportunity as any to remind everyone that the dashboard API is not thread-safe. Call it exclusively in op modes and you should be fine.)

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