Database corruption detection & repair

[wohali]

@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.