Record both the PID and the process creation-time

A new kind of pidfile in `running.process` records both
the PID and the creation-time of the process. This facilitates
automatic discovery of a "stale" pidfile that points to a
currently-running process. If the recorded creation-time matches
the creation-time of the running process, then it is a still-running
`tahoe run` proecss. Otherwise, the file is stale.

The `twistd.pid` file is no longer present.