Add a "retry" command to rejester.

With the "rejester" command-line tool, "retry -W workspec workunit" will
move "workunit" from the "failed" queue for "workspec" back to the "available"
queue; "retry -W workspec -a" will retry all failed jobs.

There is a corresponding TaskMaster.retry() API.

Registry.move() has better hygeine on its Redis Lua call, and can be
successfully and correctly called without holding a session lock.