Benchmark API

Add an API endpoint at /_bench for submitting, listing, and aborting
search benchmarks. This API can be used for timing search requests,
subject to various user-defined settings.

Benchmark results provide summary and detailed statistics on such
values as min, max, and mean time. Values are reported per-node so that
it is easy to spot outliers. Slow requests are also reported.

Long running benchmarks can be viewed with a GET request, or aborted
with a POST request.

Benchmark results are optionally stored in an index for subsequent
analysis.

Closes #60