Unnecessarily saving revisions array to metadata

[keijokapp]

I've been developing a FoundationDB adapter. FoundationDB has a hard limit on key and value lengths, so 4204 respect revs_limit brought out this issue.

It seems that _revisions passed into bulkDocs is used only to create an initial revision and tree but is then moved to document's metadata and saved to document store. The property doesn't seem to be used for anything else and removing that before returning from parseDoc doesn't break any tests. What's worse is that this array does not get reduced to revs_limit in a same way as revision tree does. So it's just there taking up space.

I'd be willing to make a PR but it seems there are also other reserved properties that are not used at all but still saved (eg. deleted_conflicts) and I'm not sure what the intentions are.