Fixes related to mutl-domain deployments

Now the external-domain-checker returns a list of local domains that can be reused between calls of ChannelManager.isLocalDomain(), ChannelManager.isLocalJid() or ChannelManager.isLocalNode(). 
It drastically improves the perfomance of disco#items, purging remote cache, etc,... and allows for a selective query of local nodes.

Since HSQLDB is not fully compliant with postgres, I had to Mockito.spy the SQL connection object in the tests in order to replace **~** with **regexp_matches**.
Why not pass the local domains through to the database query for retrieving local domains? (At most you'll have a standard and topics domain, but even that can be looped). That way it'll be quick in the DB and the code will be less complex.