Support for throttled PoW

[notlesh]

The PoW is pretty rough on a laptop :)

A couple suggestions to mitigate this:

• Ability to add some sleep() calls to throttle mining
• dev mode could support a permanent difficulty of 0 so that only a single PoW hash has to be done. this would work well with manual or instant seal.

[JoshOrndorff]

For local testing you can use --instant-seal so that you don't need to mine at all.

[JoshOrndorff]

But instant-seal sucks because you need transactions. And I think it would be nice to not need an entirely separate consensus. I think throttled PoW is the route we should take.

It could have a special flag like --slow-mine or something. But I expect the more common usage would be to have it implied by --dev and a low initial difficulty.