Add jvm-wide JMX statistics for Solr

The statistics that can currently be gathered via JMX tend to be core-specific, making monitoring "how is the Solr node doing" harder than it needs to be. This JIRA is about exploring what it would take for instance-wide statistics to be JMX-enabled.

I'm imagining cumulative stats like:
> How many Solr<->Solr communications errors have there been?
> How many Solr<->ZK communication errors have there been
> How many full synchronizations have happened across all replicas?
> Operations people, fill in your favorite health monitoring bit here.

What do people think? Is JMX even the right thing? We have an admin end-point for gathering information, but that's not as "operations friendly".

I'm open to any suggestions for how/where to implement this, whether there are any huge "gotchas", bottleneck concerns, etc.