Move all "epm:meta" data to its own file

[neonrust]

The global one, as well as for each each series.

This has several benefits.

• Update checks (and tracking) does not need to update (or even read) the main series file.
• The upcoming audit command only need to read the meta files. #38

Move all meta data, except the file version?

[neonrust]

The file should probably be named series.meta.

The meta file should be versioned. However, since we sometimes (hopefully mostly) will update only the meta file. So maybe the regular db can just be copied (or symlinked) as a new version, without writing a new file.

[neonrust]

Closed in favor of #44.