Performance monitor

Adds the Hazelcast Performance Monitor; comparable to the Health Monitor. It logs all kinds of internal metrics like read/write events per selector-thread and number of operations executed per operation-thread.

This is very useful to see progress and imbalances.

We plan to add all kinds of additional metrics over time. E.g. bytes/packets reads/written per connection/selector, event service information etc etc etc.

Another thing we'll add in the future is a mechanism to expose all this performance information through JMX. But a log file is currently for the Stabilizer tests the most practical.
Isn't it better to guard this one for the case where performance monitoring is not enabled?