4139: Speed up Node Manager tests.

Previously, the tests would poll interesting mocks, waiting for them
to be called.  This introduces significant overhead to the tests, and
they would frequently time out on Jenkins.  This modifies the tests to
get more information by blocking on the tested actors, which means
more predictability and less fighting for CPU (typical runtimes for
all the tests improved from 5 seconds to 0.5 seconds on my
workstation).

The downside to this approach is that it ties the tests more tightly
to the underlying actors' implementation.  In particular, they
sometimes send a message and block for a response to ensure that any
internal messages generated by the *last* message have been handled.
This is less than ideal, but I don't have a better idea right now.
4139: Node Manager README links to compute node lifecycle page.
Merge branch '4139-blocking-node-manager-tests-wip'

Refs #2881, #4139.  Closes #4184.