WIP: Benchmarking

[andrewosh]

Hey all,

Here's an initial stab at a benchmarking system that should help us get some solid numbers. Each benchmark is performed on 4 databases, with a customizable number of trials per benchmark (the default is 5). The initial set of databases is (and perhaps we want to add to this?):

1. hyperdb on disk
2. hyperdb in memory
3. leveldb on disk
4. leveldb in memory

The initial set of benchmarks are very simple: large batch writes, many single writes, and iterations over various subsets of a large db. The database has a single writer, and is entirely local. This set will surely need to be expanded to reflect real-world use-cases.

Speaking of real-world use-cases, all the data so far is randomly generated. @mafintosh suggested a dictionary as a more realistic dataset. Any other ideas for fixtures?

At the end of benchmarking, results are dumped into CSV files in bench/stats. Here are some examples of what those look like, from a recent run:
https://github.com/andrewosh/hyperdb/blob/benchmarking-2/bench/stats/writes-random-data.csv
https://github.com/andrewosh/hyperdb/blob/benchmarking-2/bench/stats/reads-random-data.csv
(Timing is in nanoseconds, so some post-processing is required to make it readable).

A few of things of note:

1. I'm currently using a modified version of nanobench because I started abusing it and I'm unsure if the changes I made should be reflected upstream. Before merging, that dependency (on my nanobench fork) will have to be changed.
2. The generated prefixes in the current read tests (and reflected in the above benchmarks) aren't yet split into path components -- oops. Unsure if this will affect performance, but worth noting.
3. Currently the maximum number of keys for any benchmark is 100k, since after that I'm getting consistent heap memory errors in the batch write.

[mafintosh]

there are two nanobench in the deps

[mafintosh]

@andrewosh whats missing for landing this? would be a cool addition