Add 'bolt bench'.

This commit adds a flexible benchmarking tool to the 'bolt' CLI. It allows
the user to separately specify the write mode and read mode (e.g. sequential
random, etc). It also allows the user to isolate profiling to either the
read or the writes.

Currently the bench tool only supports "seq" read and write modes. It also
does not support streaming of Bolt counters yet.

Fixes #95.

/cc @snormore