Implement miner changes for time-based tenure extensions

[obycode]

The miner needs to now keep track of the signers’ current idle timestamps and decide when it can refresh its budget with a tenure extension. A new component will process the StackerDB messages as they arrive, rather than directly in the sign coordinator. This is important because the sign coordinator stops listening for block responses from signers as soon as it hits the 70% threshold, but it is important for the miner to track the idle timestamps of all signers that report it. This component will be responsible for keeping track of the signers' latest idle timestamps, queryable from the miner. It will also provide the sign coordinator with block signatures. After each round of signing, the miner should record its estimated time to extend. It can compute this by ordering the countdown responses in ascending order, and selecting a time at which > 70% of the signing power will have passed their timestamp. Before each attempt to mine a block, check if this timestamp has passed and if so, issue the tenure extension.

See #5434