Removing nodes in testing mode should be very fast

[richardliaw]

Describe the problem

Currently, removing a node and waiting for the GCS to register the node removal takes 10 seconds.

During application testing, this is way more than needed - we should introduce some way to accelerate this (only for testing).

cc: @robertnishihara @stephanie-wang ?

[richardliaw]

Addressed in #3246

[stephanie-wang]

The config thing would be very useful on its own, but another way to make this better is to have nodes mark themselves as dead in their signal handler or mark them as dead directly from the python test. This would bypass the monitor, which currently waits 10s before marking the node as dead.

[richardliaw]

@stephanie-wang how would I do the latter?

[richardliaw]

Closed by #3246.