add endpoints for fetching metrics (without the data)

The following requests,

GET /rhq-metrics/metrics?type=num
GET /rhq-metrics/metrics?type=avail

query the metrics_idx table. We update metrics_idx whenever we insert metrics
and when a metric is explicitly created. I need to update the methods for
adding/removing meta data so that they also update metrics_idx.

This commit also switches to Jackson 2 as the json provider. I have been
trying to figure out how to suppress from output fields whose values are
null or empty. I was able to get it working with Jackson 2. We probably
want to be using Jackson 2 anyway.