work towards #131

Motivation

Before we can make significant headway improving graphsync performance, we need to have a good framework for evaluating the performance we have. This PR imports a testground graphsync test from lotus, and makes it more configurable so we can run performance tests effectively.

Proposed Changes

1. Import testplans/graphsync/main.go and associated files from Lotus.
2. Make raw nodes and chunk size configurable.
3. Add option to use a disk back store instead of an in memory store to easy collection of memory stats.