Collect statistics, reported passively via an API.

[ajtack]

Riak-Cpp should expose mean/min/max for various time periods of such values as:

• Average number of siblings encountered on get
• Number of sibling resolutions required for each successful get operation
• Numbers, separately, of
  • get requests
  • "update" puts, or those automatically generated by sibling resolution
  • "resolve siblings" puts, or those needed by the application directly
  • delete requests
• Round-trip request latency, overall
• Round-trip request latency per individual request types (not composite operations like get+sibling resolves)
• Number of simultaneous in-flight (application-layer) requests