miner: allow the miner to run only when transactions are pending

[tgerring]

Currently, the miner is with on all the time or off all the time. It would be useful in several scenarios, especially development on a private chain, to have the miner run only when transactions are pending. When the transaction pool is empty, the miner should automatically turn off and wait for more transactions.

Suggest to enable this functionality by default for --devmode flag in addition to exposing it as a discrete CLI flag.

[stale]

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