Refactor file import to the new metadata format

I have refactored the importer code. It adheres now to the new data
format I have implemented for the metadata harvester. It enabled me
to throw out a lot of code. I was also able to drop the JSON::Stream
dependency.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Revert tag accessor path

Since I am back to flatten the fields of groups and tags, I can also
revert the tag accessor path as used by the Richness of Information
metric.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Change domain model by introducing snapshot entity

It becomes increasingly hard to get the right data when having more than
one snapshot, because it consumes too much time to run over all metadata
of a repository. Since I am interested only in one metadata import at a
time I think it is best to introduce a new abstraction here: snapshots.

A snapshot groups together a list of metadata records associated to the
repository and with a certain date (the harvesting date).

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Update code to fetch metadata through snapshots

I had to change a lot of code to make this working again. I also renamed
the metadata class into metadata record.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Fix display of last updated field

I had to update the scheduler helper to fetch the last updated
information, too.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>