Occasionally, out of order notifications received from a resize operation
would incorrectly produce a verification error, because the launched_at time
is changed several times during the operation.
Fix this to keep the last chronological launched_at time in the operation,
not the last received.
Also fix nondeterministic multiprocessing bug that was occasionally causing
unittests to hang.
Change-Id: Iba8b0bbd0cb8b2b063335ca9ab0ad95cf127087a