Better end-to-end integration tests

[paddycarver]

We currently test all the pieces of the algorithm, but don't have automated tests for the overall algorithm itself. If Wendy is ever going to exit alpha status and be usable in production, we need better tests that show that the algorithm itself works as expected, in addition to all its component parts.

[paddycarver]

As of the beta1 release (specifically, commit afb3699), we now have integration tests for nodes joining the cluster. We still need race condition tests, message routing tests, and tests for nodes exiting the cluster.