Improve stats during partition healing #143
Description
We should add more stats to partition healing.
For healing through discover provider:
- timer for complete heal-operation
- counts for success heal-operation
- counts for error (max failures) heal-operation
For heal-attempts:
- timer for complete heal attempt.
- count for successes
- count for errors
(this is the ringpop-go counterpart of uber-node/ringpop-node#267)