Add couch_db_header:latest/1
Add couch_db_header:latest/1

This function returns false if it's supplied an integer less than the
?LATEST_DISK_VERSION, true if supplied ?LATEST_DISK_VERSION, and
undefined on any other input.

BugzID: 15762

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