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/client/api/src/in_mem.rs

@@ -369,6 +369,7 @@ impl<Block: BlockT> HeaderBackend<Block> for Blockchain<Block> {
 				None
 			},
 			number_leaves: storage.leaves.count(),
+			block_gap: None,
 		}
 	}