Add more useful ceres plugins.

[iain-buclaw-sociomantic]

• merge.py: Merge any slices that have overlapping data.
• metadata.py: Update node metadata to match carbon schemas.
• orphans.py: Roll-up slices on disk that don't match any retentions.
• rollup_ng.py: Better, more efficient rollup plugin.

These are used in my production environment, haven't had any problems with them in the two or so years they've been in service.

Closes #6

[deniszh]

That's a nice addition, thanks, Ian!

[deniszh]

Codacity complaining for couple of issues, meh.. OK, merging for now.

[iain-buclaw-sociomantic]

Codacity complaining for couple of issues, meh.. OK, merging for now.

What are the issues?

[deniszh]

General 'except' w/o error, see above. Not sure if relevant, though.

[iain-buclaw-sociomantic]

I had a quick look, and it seems that there can only be one exception that's caught from those locations, putting out PR.