fix(sync): allow estimation errors (#329)

Another tail determination bug again. 

When we have both old tail and head, we look into the stored local
header timestamps to mitigate estimation errors. However, the assumption
was that we could always get the header locally, which is not the case.
If the node was offline for long enough, it may be possible that the new
estimated tail is above the old head, causing header loading to deadlock
on heightSub.

The fix here ensures that header loading is only within the current
boundaries of the store, and if not, it proceeds with the estimate
without checking for errors.

Author: Wondertan

sync/syncer_tail.go

@@ -103,7 +103,15 @@ func (s *Syncer[H]) renewTail(ctx context.Context, oldTail, head H) (newTail H,
 		log.Debugw("fetched tail header by height", "height", tailHeight)
 	}
 
-	if err := s.store.Append(ctx, newTail); err != nil {
+	err = s.store.Append(ctx, newTail)
+	var errNonAdj *errNonAdjacent
+	if errors.As(err, &errNonAdj) {
+		// TODO(@Wondertan): We have to force through the write here
+		//  We don't car if its non-adjacent
+		//  Remove with bsync
+		err = s.store.Store.Append(ctx, newTail)
+	}
+	if err != nil {
 		return newTail, fmt.Errorf("appending tail header %d: %w", newTail.Height(), err)
 	}
 
@@ -235,9 +243,9 @@ func (s *Syncer[H]) findTailHeight(ctx context.Context, oldTail, head H) (uint64
 	)
 
 	newTailHeight := estimatedTailHeight
-	for {
+	for newTailHeight > oldTail.Height() && newTailHeight < s.store.Height() {
 		// store keeps all the headers up to the current head
-		// to iterate over the headers and find the most accurate tail
+		// iterate over the headers and find the most accurate tail
 		newTail, err := s.store.GetByHeight(ctx, newTailHeight)
 		if err != nil {
 			return 0, fmt.Errorf(