DataMananger - Add fallback opt-out and user defined prefered adapters. ...

this is for https://issues.jboss.org/browse/AGJS-93

I've added 2 new config settings to Datamanager.

1. fallback - by default this is true, but if you want to opt-out of falling back to another adapter, the set this to false

2. prefered - you can specify what the fallback order should be,  defaults to `[ "IndexedDB", "WebSQL", "SessionLocal", "Memory" ]`