Database corruption detection & repair

@janl:
> 2012 Dev Summit leftover

@davisp:
>I'd probably move this to a bullet point under pluggable storage engines where new experimental engines could add singificantly more error checking/detection/correction code and then it would be a matter of "let the best storage engine win" sort of thing.

@wohali:
>I'm avoiding merging all of this stuff into other tickets, they then become too big grained and we never get anything done.

Also we still need a solution for couch_file corruption detection and repair that is easy to manage. (Right now it is available as a paid consulting offering from Neighbourhoodie.

The issue names couch_file but provides no specific source files, tests, entry point, or acceptance criteria. First map the existing corruption detection and repair path in CouchDB; done would require an agreed implementation scope and a way to validate both detection and repair.

Written by the indexing model from the issue text.