[miner] reduce the subsequent attempt time for mining

[jcnelson]

See if we can make the default subsequent block timeout smaller, given the MARF speed improvements. In particular, see how quickly a miner can pack a full block, and use that to inform us on what default value to have.

[jcnelson]

I went through all of the blocks the Foundation miner has assembled during and after last week's high-congestion period. In no case did it take longer than 30s to assemble a block (in fact, the highest time was about 27s). Going to send a PR to make this the new default subsequent-attempt value.

[blockstack-devops]

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