benchmarks: Add setup, bench naming, and custom count default

benchmarks: Add setup, bench naming, and custom count default

* bench(): Add support for setup function.
  Demonstrated by converting bench_delete_truncate.php to use Benchmarker.

* bench(): Allow benchmarks to be named. Default remains (fn + args).
  Useful for closures.

* Benchmarker: Support overriding the default count of 100.
  Demonstrated in bench_delete_truncate.php to run 10x instead of
  100x (previous: 1x).

Change-Id: Iac182eaf3053f5bf0e811cd23082f530629d8a4e