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We use these outside of bitswap for testing.

This makes reading the docs much easier as it's clear what's "private" and
what's not.

fixes #238

per https://github.com/golang/go/wiki/CodeReviewComments#comment-sentences

- add a delay generator that similates real world latencies one might encounter on the internet
- modify virtual network to accept different latencies for different
peers based on using NextWaitTime on passed delay
- modify dup_blocks_test subtestDistributeAndFetch to accept a custom
delay
- Add a real world benchmarks that simulates the kinds of problems one might
encounter bitswaping with a long lived session and a large swarm of
peers with real world latency distributions (that causes #8 not to
function well in practice)