Graceful *restart* of the master in addition to existing graceful shutdown

[dustin]

exarkun would like to be able to click something like the graceful-shutdown button on the master, with the expectation that it will restart when the shutdown is complete.

This would be similar to the reconfig-by-graceful-restart support I hope to have sometime in the 0.10.+ timeframe.

It may be worth building out a system where the buildmaster is composed of a parent "supervisor" process that can manage this sort of activity. It may even be worthwhile to implement this by essentially subclassing the twistd command-line tool (rather than invoking it)

[dustin]

dup #994