Interrupt miner when Stacks tip changes

[obycode]

This is an optimization to improve the time it takes to mine the first valid block in a tenure. When mining a tenure, the miner thread should get interrupted when the tip advances. The way it currently works, at the beginning of a tenure, the miner will often start mining its first block before it sees the last block of the previous tenure. If that last block is accepted and the miner's tip advances, the miner finishes building the block and proposing it to the signers any way. Instead, the miner should stop building its current block, or stop waiting for signatures, since it knows that it will be rejected by 70+% of the signers for not confirming that last block. It can then quickly build a new block atop the new tip.