the timeouts on uses of 'poll' were there purely to make sure a test doesn't
poll indefinitely. however having such timeouts makes tests susceptible
to premature timeouts under high load, or on slow machines. (e.g. cygwin
slaves running in virtual machines on loaded hosts)
purportedly trial by default applies a timeout to tests to prevent them
hanging out indefinitely, so these poll timeouts are redundant and cause
intermittent failures on slow hosts. hence they're more bother than they're
worth, and should be culled.
in both these cases, the timeout only serves to abort a stuck test, and
the key_generator should respond more quickly, but seeing test failures
in buildbot on some platforms suggests that the test is too susceptible
to timing issues on loaded buildslaves.
this cleans up KeyGenerator to be a service (a subservice of the
KeyGeneratorService as instantiated by the key-generator.tac app)
this means that the timer which replenishes the keypool will be
shutdown cleanly when the service is stopped.
adds checks on the key_generator service and client into the system
test 'test_mutable' such that one of the nodes (clients[3]) uses
the key_generator service, and checks that mutable file creation
in that node, via a variety of means, are all consuming keys from
the key_generator.
