Add counters to track nodestore read and write activities.

@JoelKatz @HowardHinnant please review.

This adds read and write counters, as well as total data sizes (not including hash key) read and written, for each activity to the nodestore. Results are currently emitted via get_counts. This format can plug easily into any stats display system by measuring differences over time to derive throughput.

The goal is to be able to measure nodestore activity in general as well as to measure affect of rotate-delete on the nodestore. We can also see the effectiveness of key-value store caching behavior by comparing these numbers to system physical I/O statistics.

Overhead should be minimal here. Up to two additions to atomic unsigned ints for every store and every fetch.