Run plot comms on the R thread

[lionel-]

Progress towards #689
Branched from #1099

I did not foresee all the simplifications falling out of this one!

• Plots no longer live in their own threads, they live on the R thread. That frees up memory since each background thread takes about 2mb of stack space (depending on platform), so 20 open plots would take 40mb of memory.

• DeviceContext moves from a standalone thread_local! into a regular field of Console. All access goes through Console::get().device_context(). The CommHandlerContext provides access to Console via ctx.console(), so plot handlers reach the device context through a controlled context chain.

• The old idle-time polling loop (process_rpc_requests with crossbeam::Select) is removed. Plot RPCs are now dispatched synchronously through the shell handler on the R thread. This improves plot latency when pre-renderings require adjustments because we no longer timeout-poll for plot updates (related to Performance for plotting positron#5184).

• The GraphicsDeviceNotification async channel is removed. Since the UI comm now runs on the R thread (from Run UI comm on the R thread #1099), prerender settings are updated synchronously via Console::get().device_context().set_prerender_settings(). This removes quite a bit of plumbing.

• Fixes a dev.hold() regression I introduced with pre-renderings (Send pre-renderings of new plots to the frontend #775). We were previously sending pre-renderings unconditionally, now they are held until dev.flush(). This is tested by new integration tests.