Light-client Authority Set Handoffs

[gavofyork]

[rphmeier]

This is desired for light-client-friendly tracking of authority set changes.

When the authority set changes from A to A' at a block B, The header of B will contain an NewAuthorities(A') event in the digest.

When an authority imports such a block and believes the change to be the result of a valid block execution, he generates a signature on a HandoffAttestation(Hash(B), NewAuthorities(A')) message and circulates it to other authorities. Upon witnessing such signatures from a majority of authorities, he accumulates them into a full Justification(B, NewAuthorities(A')) message.

When a full or light node imports a block with this kind of digest event, it waits to fetch a justification for the change before continuing to sync on top of that chain. Full nodes will save the justification permanently in the database.

Optimization: automatically provide the justification with any relevant blocks.

This can also be used for a fast warp sync with the caveat that it is vulnerable to long range attacks.

[gavofyork]

Might be a good one for @svyatonik ?

[gavofyork]

Done in #687

[svyatonik]

Not actually - we only have a DigestItem trait now - everything else is not ready yet.

[svyatonik]

Current state:
#687 : DigestItem::AuthoritiesChange
#726 : DigestItem::AuthoritiesChange is emitted when authorities set changes
1d0f52f : (probably we don't need this at all) 'pausable' synchronization

TODO : generate && broadcast justification by authorities. Since (as @rphmeier said) this is required for non-instant finality, I'll put this away for now