Add database latency reporting to Crowd

Issue Summary

It would be nice if Crowd had a database latency check similar to what Confluence has in it's 'System Information' page, which performs the following test to return a value:

The latency is calculated by sending a trivial request to the database, querying a table which is known to have only one column and one row. ("select * from CLUSTERSAFETY"). Obviously this query should be blazing fast, and return within 1 or 2 milliseconds.

From https://confluence.atlassian.com/doc/performance-tuning-130289.html

Note on the implementation
The latency measurement is also added to the Confluence support zip. It's available in application-properties/application.xml under the <stp.properties.db.example.Latency> element.