[miner] Miner should issue a tenure extend if the incoming miner fails to produce a block proposal

[jferrant]

Signers will already accept tenure extends from an previous miner if the current miner is marked InvalidatedBeforeFirstBlock (this occurs if the current miner fails to send a single block proposal within block_proposal_timeout_ms which is defaulted to 10 minutes)

The previous miner should watch for block proposals from the incoming miner and if it doesn't see one/stacks chain tip doesn't advance etc. within some timeout, should attempt to tenure extend.

Could in theory not even bother with observing the miner's behaviour and if the stacks tip just doesn't advance within 10 minutes of the incoming miner's tenure starting...issue a tenure extend, but this could result in a prior miner issuing tenure extends that would never get accepted (for example, the incoming miner could have proposed an invalid block proposal immediately, but this would prevent the prior miner from ever being able to extend regardless of the fact that the signers rejected the incoming miner's proposal and the tip has not advanced)