Support a global flag file to exclude some snapshots from cleanup

The greatest problem with BTRFS in terms of backing up to an external disk is that if there is no common snapshots on both sides, then you have to start all data transfer from scratch.

The backup rotation feature is a MUST, we need to keep a limited number of snapshots in our computers.

If we have too many snapshots taken on our host system, then every common snapshot with the external disk is eventually lost. How can we prevent this case from happening?

Simple solution is to mark the latest snapshot's timestamp by writing it to a global `filter` file so that any `btrbk` instance will skip deleting any snapshot having that timestamp upon backup rotation.

Improvement: If `btrbk` would check the contents of all files in a filter folder (like `/etc/apt/sources.d/`), then any script/implementation can register and deregister its "DO_NOT_DELETE snapshots" easily.