Capture additional JMX metrics for LogMiner

It would be useful to capture the following metrics:

• The duration it took to begin the current log mining session
• The lowest & highest duration it took to begin log mining sessions
• The current number of redo logs in the mining session
• The minimum/maximum number of redo logs used for a mining session
• The duration of the last full mining session (start and end)
• The shortest & longest duration of a full mining session

Those marked with  have already been added in other work.