Add sensors to report the number of slow brokers

[efeg]

This PR resolves #1210.

1. Adds the following sensors:

DESCRIPTION	MBEAN NAME
The number of brokers that are metric anomaly suspects, pending more evidence to conclude either way	kafka.cruisecontrol:name=AnomalyDetector.num-suspect-metric-anomalies
The number of brokers that have recently been identified with a metric anomaly	kafka.cruisecontrol:name=AnomalyDetector.num-recent-metric-anomalies
The number of brokers that continue to be identified with a metric anomaly for a prolonged period	kafka.cruisecontrol:name=AnomalyDetector.num-persistent-metric-anomalies

2. (Trivial) Make sensor constant names public to enable access to them from outside the class.

[Lincong]

Looks good to me!