Node needs JIT pending header computation

[wizeguyy]

Presently if the node stops, the pending headers are forgotten. They also cannot be reliably stored before the node stops -- see #476 for information on that. Ultimately, we need to have a way to compute a pending header just-in-time when requested.

User story:

1. I have a fully synchronized full node
2. I stop my node and restart it
3. My node should immediately able to provide a pending header to the miner