mesh: remove global timeout thread

[jstarks]

For convenience, mesh cancel contexts are timed out via a global timeout thread. Find a better way to manage this, e.g.:

• Pass a pal_async driver to until_cancelled. This isn't great because we want to avoid a pal_async dependency in the public API.
• Require the platform to be initialized with an explicit driver for handling timeouts. This works OK but means every program using mesh has to remember to do this in main.
• Grab the driver from the current task. This seems easy to screw up since we're not always running on a task.

None of these solutions is obviously the right one. But it's awkward to have this extra thread.