epoch-changes: separate epoch header with epoch data (#4881)

* fork-tree: prune returns all pruned node data

* epoch-changes: split EpochHeader vs epoch data

* EpochChanges::viable_epoch and add missing comments

* Incoperate the new epoch_changes interface for BABE

* Fix BABE tests

* Fix fork-tree pruning issue

* Fix tests

* Fix pruning algorithm

* fork-tree: implement map function for mapping one value type to another

* Add migration script for new epoch changes scheme

* Update utils/fork-tree/src/lib.rs

Co-Authored-By: André Silva <andre.beat@gmail.com>

* Update client/consensus/slots/src/lib.rs

Co-Authored-By: André Silva <andre.beat@gmail.com>

* Remove authorities_len.is_none check, which is duplicate of unwrap_or(false)

* Update client/consensus/epochs/src/lib.rs

Co-Authored-By: André Silva <andre.beat@gmail.com>

* Update client/consensus/epochs/src/lib.rs

Co-Authored-By: André Silva <andre.beat@gmail.com>

* No trailing ; for return statement

* Use VERSION_KEY for migration

* Fix issues that removed nodes are not properly added into removed list

* Add comments indicating end_slot is non-inclusive

* fork-tree: use &mut F for map type declaration

* Add tests for v0 epoch_changes migration

* Fix babe RPC tests

Co-authored-by: André Silva <andre.beat@gmail.com>

substrate/client/consensus/aura/src/lib.rs

@@ -230,8 +230,8 @@ impl<B, C, E, I, P, Error, SO> sc_consensus_slots::SimpleSlotWorker<B> for AuraW
 		authorities(self.client.as_ref(), &BlockId::Hash(header.hash()))
 	}
 
-	fn authorities_len(&self, epoch_data: &Self::EpochData) -> usize {
-		epoch_data.len()
+	fn authorities_len(&self, epoch_data: &Self::EpochData) -> Option<usize> {
+		Some(epoch_data.len())
 	}
 
 	fn claim_slot(