This helps determine which blocks are unlikely to end up as uncles

 * Store the 5 most recent locally mined block numbers
 * On every imported block, check if the 5-deep block num is in that store
 * Also confirm that the block is signed with miner's coinbase

Why not just check the coinbase? This log is useful if you're running
multiple miners and want to know if *this* miner is performing well.