Warp sync part II (#9284)

* Gap sync

* Gap epoch test

* Simplified network requests

* Update client/db/src/utils.rs

Co-authored-by: cheme <emericchevalier.pro@gmail.com>

* Fixed v1 migration and added some comments

* Next epoch is always regular

* Removed fork tree change

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Added a comment and converted assert to error

Co-authored-by: cheme <emericchevalier.pro@gmail.com>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/primitives/blockchain/src/backend.rs

@@ -281,6 +281,8 @@ pub struct Info<Block: BlockT> {
 	pub finalized_state: Option<(Block::Hash, <<Block as BlockT>::Header as HeaderT>::Number)>,
 	/// Number of concurrent leave forks.
 	pub number_leaves: usize,
+	/// Missing blocks after warp sync. (start, end).
+	pub block_gap: Option<(NumberFor<Block>, NumberFor<Block>)>,
 }
 
 /// Block status.