Refactoring `ethcore-sync` - Fixing warp-sync barrier (#8543)

* Start dividing sync chain : first supplier method

* WIP - updated chain sync supplier

* Finish refactoring the Chain Sync Supplier

* Create Chain Sync Requester

* Add Propagator for Chain Sync

* Add the Chain Sync Handler

* Move tests from mod -> handler

* Move tests to propagator

* Refactor SyncRequester arguments

* Refactoring peer fork header handler

* Fix wrong highest block number in snapshot sync

* Small refactor...

* Address PR grumbles

* Retry failed CI job

* Fix tests

* PR Grumbles