use couch_db calls instead of couch_btree
use couch_db calls instead of couch_btree

couch_changes can use the interface from couch_db to access documents,
keeping the logic of the btree storage better encapsulated.

fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb