Add sys db callbacks only once per db open
Add sys db callbacks only once per db open

Currently we are adding the sys db callbacks for every request instead of
once while opening the db. This fix will ensure that we will only add the
sys db call backs once while opening the db.

COUCHDB-3307

apache/couchdb-couch,apache/couchdb-couch,fkaempfer/couchdb,apache/couchdb-couch,fkaempfer/couchdb,cloudant/couchdb-couch,cloudant/couchdb-couch,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,cloudant/couchdb-couch,fkaempfer/couchdb