Model testing with integration tests

[Stebalien]

Many of our tests simply check to make sure that we reach the correct result. However, we don't often check to see if go-ipfs reached the correct result the correct way.

We should add some model testing by turning on debug logging for specific subsystems, executing a simple integration test, and checking to see if we get the expected result.

For example, we should verify that every gateway request uses a single bitswap session. We can verify this by making a bunch of independent gateway requests, logging when new sessions are created, and checking to see if we're creating too many sessions.