Improve local caching and use report-by-exception when sending metadata back

[knolleary]

Currently we post back topic information whenever we see it arrive (with some local caching to do bulk updates).

We don't currently make use of the 'last seen' field of the meta data, nor do we have plans for it.

I propose we update the agent to cache locally everything it sees and do more of a report-by-exception; only post back to the platform when something is new or the detected schema has changed.

That will reduce the load when pointed at a busy broker.

[hardillb]

The local cache should pre-populate from any existing topics in the database for this set of credentials.