rebased _bulk_get patch

This is a rebase for https://issues.apache.org/jira/browse/COUCHDB-2310 against post-1.6.x CouchDB.

The idea is that when it is finished *every* GET option should eventually be supported in the _bulk_get API, making CouchDB easier to learn than a hypothetical _bulk_get API supporting only a subtly different subset of the GET API.

Is it acceptable to refactor the GET handling?

Is anyone willing to help get this polished accepted into CouchDB?

I wrote this patch because without it CouchDB replication to PouchDB is too slow to be useful. After this patch, formerly impractical replications that would take ~10,000 requests now take a pleasantly acceptable ~100 requests.

https://github.com/dholth/pouchdb-bulk-get is a PouchDB plugin for the other end.

Thanks.