Miner

This refactors the actual piece that gives headers to the mining clients. I changed a few things and added a bunch of tests. There were a few bugs I found that could have been the source of the high stale rate.

It's hard to be certain that the stale rate bug has been fixed, becuase it's hard to set up a dev environment with multiple competing GPUs in a realistic latency-environment. But the testing is a lot more complete, and I imagine the stalerate will be much closer to where it should be.