performance and correctness updates to miner

During IBD and loading, some hashing in the miner module was causing visible slowdown. While investigating other potential sources of slowdown, I also discovered some locking misuse.

Removing the hashes resulted in about a 10% speedup for the miner accepting an average sized block (where average was defined by using the real chain). FindBlock had an outright deadlock in it. An unchecked error is now checked because the miner now has a logger that can record the error.