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,cloudant/couchdb-couch,KlausTrainer/couchdb-couch,ngpestelos/couchdb-couch,smithsz/couchdb-couch,smithsz/couchdb-couch,KlausTrainer/couchdb-couch,smithsz/couchdb-couch,ngpestelos/couchdb-couch,fkaempfer/couchdb,cloudant/couchdb-couch,apache/couchdb-couch,fkaempfer/couchdb,apache/couchdb-couch,apache/couchdb-couch,alanning/couchdb-couch,fkaempfer/couchdb,cloudant/couchdb-couch,fkaempfer/couchdb,fkaempfer/couchdb,ngpestelos/couchdb-couch,KlausTrainer/couchdb-couch,alanning/couchdb-couch,alanning/couchdb-couch