Persistent Headers

[chjj]

We should be saving the last 1152 headers to disk. Flat file storage can work for this. Every hour or so, we can flush the current state to a single file and read it again on boot. It's a couple hundred kb to write every hour, which shouldn't be a big deal.

Having this ready by v1 is ideal, but realistically, this only needs to be ready a couple months after the chain is launched. Headers are fast to sync. It will only become a pain later on.