Rewrite how statistic are exposed for consumption

[tkowalcz]

After implementing current approach to doing metrics I have identified some drawbacks to how these are exposed.

To access the metrics user has to get hold of the appender object (through ((org.apache.logging.log4j.core.LoggerContext) LogManager.getContext(false))) and set MonitoringModule implementation.

Proposal is to add configuration entry that will enable statistics in the appender as well as specify a transient file (possibly in/dev/shm) that will be mmaped and used to dump statistics in real time. We will provide utility classes to decode that file and send the stats via e.g. dropwizard metrics.