Sync block justifications (#1410)

* core: sync protocol for justifications

* core: basic test for justification sync

* core: pass block number with justification

* grandpa: request justifications when importing change blocks

* core: pass finality notifications to chain sync

* core: require justifications for pending change blocks on start

* core: avoid requesting justifications from previous failed peers

* core: timeout block justification requests

* core: add some docs

* core: fix unused variables warning

* core: tick pending justifications fetch periodically

* grandpa: add test for syncing justifications

* core: early exit dispatch of pending justifications

* core: style fix

* core: grandpa: change logging level

* core: sync: add missing docs

* core: network: report peer on bad justification

* core: replace mem::replace with Option::take

* core: revert authority set changes on failed block finalization

* core: grandpa: add docs to import_justification

* core: warn on re-finalization of last finalized block

* core: only notify sync with last finality notification

* core: style fix

* core: add docs for PendingJustifications

* core: network: use BlockRequest messages for justification requests

* core: reference issues in todo comments

* core: grandpa: revert authority set changes on db

* core: grandpa: remove inconsistent state warning

substrate/core/finality-grandpa/src/authorities.rs

@@ -146,7 +146,6 @@ where
 	}
 
 	/// Inspect pending changes.
-	#[cfg(test)]
 	pub(crate) fn pending_changes(&self) -> &[PendingChange<H, N>] {
 		&self.pending_changes
 	}
@@ -261,7 +260,7 @@ pub(crate) struct PendingChange<H, N> {
 
 impl<H, N: Add<Output=N> + Clone> PendingChange<H, N> {
 	/// Returns the effective number this change will be applied at.
-	fn effective_number(&self) -> N {
+	pub fn effective_number(&self) -> N {
 		self.canon_height.clone() + self.finalization_depth.clone()
 	}
 }