Expose ontology metrics via JMX

[arteymix]

Ontology search and inference is critical for the browser performance. I suggest that we keep track of some relevant metrics:

• [x] timers for getChildren() and getParent()
• [ ] initialization time and reinitialization time too
• [ ] number of terms and statements in each ontology
• [ ] active status of ontologies (loaded and enabled)

[arteymix]

I've added timers in c698ae90a92d244f47aa3ab46227807331e7ed80.