Updates to /status API

This patch returns additional data from the /status API call. Specifically, it
returns a worker list, the message broker connection status, the database
connection status, and the Pulp version (RHBZ #704184).

There is a small behavior change to include the resource manager and scheduler
as part of the worker heartbeat list. Previously, these were not included in
the list.
You wouldn't have to do this if there wasn't a StatusManager object and all its methods were just module-level functions instead.