Added status for datasources and exposed status via API

This change adds get_status() for every datasource and
exposes that information through the API.  Right now,
the status of a datasource consists of (i) the time
the service was last updated and (ii) the last
error that occurred.

Implements: dse-dataservices-status
Change-Id: I75271dc82c7889061f9d125d9205134a0daea817
Merge "Added status for datasources and exposed status via API"