Add SeenTable to avoid continuous attempts to dead peers.
Refactor onSecond.
Block backward sync while forward sync is working.
SyncManager now checks responses according corresponding requests + tests.
SyncManager now watching for not progressing local_head_slot and resets SyncQueue.
* Add ability to reset state of sync manager.
Fix bug when sync got stuck on `zero-point` reset.
Fix bug when sync got stuck when some of the workers waiting for failing one.
* Remove debugging comments and imports.
* Remove not used pendingLock.