Publish more stats when learner gets a diff, full snapshot, or does a truncate when it connects with the leader

Problem
There is no way to currently tell whether a learner gets a diff or full snapshot, or does a truncate when it connects with the leader, also how long syncing with the leader takes. There is no explicit and general indicator of how often a learner has to connect to a leader without greping through zookeeper.log

Solution
Start tracking and exporting the following three items:

• counter incremented each time a learner sync's with a leader
• the type of sync that was needed
• how long the sync took