Ensure nmc2couch/PouchDB is not appending changes to entries that have not actually changed.

[indolering]

nmc2couch basically takes Namecoin entries and dumps them into an internal database that it then replicates to CouchDB. Since nmc2couch checks multiple blocks on every pass and checks all domains once every 12-24 hours, we need to ensure that any domains that have not changed are not being saved as revisions (and thus wasting space).