Collect and log performance metrics

[psFried]

When the old storage engine was replaced, performance logging went from kind of crappy to non-existent. This issue is to track re-implementing the collection of performance metrics, and serves as a place to collect requirements.

Operations to collect metrics on for the first pass:

• append time from the partition's perspective
• End-to-end time to append events from the connection handler's perspective
• Time to create a reader in the backend partition
• End-to-end time to start consuming from the connection handler's perspective
• Every operation time spent waiting in the channel
• Time to go from uncommitted to committed when appending an event

Nice to haves:

• Ability to disable or throttle down perf logging
• Ability to customize aggregation parameters
• Ability to set thresholds for logging particular events
• Extensible perf metrics output (statsd, etc.)
  • For the initial implementation, it's probably fine to just use the normal logger