Improve stats during partition healing

[mennopruijssers]

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-node counterpart of uber/ringpop-go#143)