Avoid switching CPUs in epoch_drain_callbacks().

[markj]

Summary

This avoids problems caused by binding to a CPU monopolized by a
high-priority thread.

Instead, implement a simple state machine that allows callback threads
to signal when they're done executing deferred callbacks for a given
epoch domain.

[markj]

I understand that there is a plan to remove this interface entirely, but this provides a workaround in the meantime and could be merged to 12.

[• hselasky]

I need some more time to review this. Any updates?

[markj]

In D24621#548401, @hselasky wrote:

I need some more time to review this. Any updates?

I have no updates. I'll spend some more time on testing shortly.