miner: immediately include uncles in pending block

[cdetrio]

This fixes #2902

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[karalabe]

We need to rework the miner fist to permit multiple work blocks at the same time. Otherwise this would bump up the invalid share count on pools quite heavily since this invalidates the current works and starts new one. The solution seems ok btw.

[chfast]

@karalabe, do you mean to rework the ethminer?

[chfast]

Check out this ethminer build: ethereum-mining/ethminer#217.

[cdetrio]

What @karalabe might mean by "rework the miner" is to make sure the SubmitWork handler in remote_agent.go holds onto previously constructed pending blocks. I think it does, the only edge case I'm aware of is when two or more blocks are found at the same height (i.e. one mainchain block and then an uncle block), this delete(a.work, hash) will prevent the second submission (a found uncle block) from being accepted.

[karalabe]

Closing this being the wrong approach, but I'll get it addressed in Gary's rework :)