Add API to get doc body given its bp (file offset)

To efficiently store CouchDB-style revision trees in CouchStore, we need a way to get to an older revision of a document, at least until the file is compacted. The simplest way to do this is to get the current revision's DocInfo.bp (file offset) and store that value as part of the new revision's body. Then we just need an API to get a document body given a bp value, which is pretty trivial.