node: use BABE predigest data to find authorship

[andresilva]

We already had all the infrastructure in place to collect authorship data from BABE predigests but it wasn't wired in on the node runtime.

Fixes #2986.

[andresilva]

I changed the FindAuthor session adapter to use a u64 for authority index because that's what BABE uses. But maybe it's more reasonable to instead have BABE use a u32 instead.

[kianenigma]

sweet.

[rphmeier]

I changed the FindAuthor session adapter to use a u64 for authority index because that's what BABE uses. But maybe it's more reasonable to instead have BABE use a u32 instead.

We've bounded the length of authority lists by u32 elsewhere, so I don't think this would be controversial.

[andresilva]

Updated BABE to use u32 for authority index, this breaks sync with FF but there are more breaking changes on the pipeline and we will reset FF shortly anyway.