Enable incremental updates for process dumping

[GeigerJ2]

The current behavior of verdi process dump allows users to dump processes as they are still running. In that case, however, not all files that eventually end up being involved in the process once it's finished are being dumped (obviously). When running the command again when the process is finished, the user needs to use the --overwrite flag to re-create the directory, otherwise the command fails.

It would be nice if, instead, incremental updates would be possible. This is also a feature that will be necessary for the intended verdi profile sync command that allows the user to incrementally dump all data of a profile to disk. It could be achieved either by writing the ProcessState or mtime to a file, and to check against that.