replace Twistd plugins with new `twist` tool

[warner]

Twisted-16.4.0 was just released, and includes a command-line tool named twist (to replace the previous twistd). The internals of this tool appear to be better suited for running programmatically. We currently use an awkward fake plugin to use the twistd functionality inside tahoe start.

This ticket is to investigate whether we can simplify that module by using twist (specifically twisted.application.twist._twist.Twist) instead of twistd. Note that twist may be entirely internal, and thus off-limits for non-Twisted code to depend upon, in which case we'll have to close this ticket as a WONTFIX.