(#132) - Get rid of 'dbs' by using allDbs() more

Simplifies the code a bit - mostly because the _users and _replicator dbs no longer have to keep the dbs cache up to date all the time. Should make the upcoming refactoring easier, as far as I can see this was the largest road block for that.

Tested against couchdb-harness (was fine except for the unrelated http://paste.ubuntu.com/9304254/), test-pouchdb & did some manual testing.