The "key-generator" node type has been removed. This was a standalone process
that maintained a queue of RSA keys. Clients could offload the key-generation
work by adding "key_generator.furl=" in their tahoe.cfg files, to create
mutable files and directories faster. This seemed important back in 2006, but
these days computers are faster and RSA key generation only takes about 90ms.

This removes the "tahoe create-key-generator" command. Any
"key_generator.furl" settings in tahoe.cfg will log a warning and otherwise
ignored. Attempts to "tahoe start" a previously-generated key-generator node
will result in an error.