the up2k databases are, by default, stored in a `.hist` subfolder
inside each volume, next to thumbnails and transcoded audio
add a new option for storing the databases in a separate location,
making it possible to tune the underlying filesystem for optimal
performance characteristics
the `--hist` global-option and `hist` volflag still behave like
before, but `--dbpath` and volflag `dbpath` will override the
histpath for the up2k-db and up2k-snap exclusivey
e1b9ac631f