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.
this adds a new service to pre-generate RSA key pairs. This allows
the expensive (i.e. slow) key generation to be placed into a process
outside the node, so that the node's reactor will not block when it
needs a key pair, but instead can retrieve them from a pool of already
generated key pairs in the key-generator service.
it adds a tahoe create-key-generator command which initialises an
empty dir with a tahoe-key-generator.tac file which can then be run
via twistd. it stashes its .pem and portnum for furl stability and
writes the furl of the key gen service to key_generator.furl, also
printing it to stdout.
by placing a key_generator.furl file into the nodes config directory
(e.g. ~/.tahoe) a node will attempt to connect to such a service, and
will use that when creating mutable files (i.e. directories) whenever
possible. if the keygen service is unavailable, it will perform the
key generation locally instead, as before.
