Add script to purge changed files from cache

This maintenance script enhances the prior purgeDeletedFiles script to
optionally also process create and modify events that may have left
stale information in front-end caches.

Changes from purgeDeletedFiles:
* `--starttime` & `--endtime` are now required
* `--type` allows selecting 'created' and/or 'modified' events in
    addition to 'deleted' events.
* `--dry-run` to list affected files but perform no purges
* `--htcp-dest` option to restrict HTCP broadcast messages

Change-Id: I7181ea461ba9758747bff4fd70530d00dab492cc
Merge "Add script to purge changed files from cache"