key_generator: fix timing, make tests more robust

key_generator: fix timing, make tests more robust

previously there was an edge case in the timing of expected behaviour
of the key_generator (w.r.t. the refresh delay and twisted/foolscap
delivery).  if it took >6s for a key to be generated, then it was
possible for the pool refresh delay to transpire _during_ the
synchronous creation of a key in remote_get_rsa_key_pair.  this could
lead to the timer elapsing during key creation and hence the pool
being refilled before control returned to the client.

this change ensures that the time window from a get key request
until the key gen reactor blocks to refill the pool is the time
since a request was answered, not since a request was asked.
this causes the behaviour to match expectations, as embodied in
test_keygen, even if the delay window is dropped to 0.1s