Dont reopen idx with refs (#12346)

It looks like the closeIdx function here will close indexes while they
are referenced by transactions.

This adds a check to stop that.

turbo/snapshotsync/freezeblocks/block_snapshots.go

@@ -354,6 +354,10 @@ func (s *DirtySegment) reopenIdxIfNeed(dir string, optimistic bool) (err error)
 }
 
 func (s *DirtySegment) reopenIdx(dir string) (err error) {
+	if s.refcount.Load() > 0 {
+		return nil
+	}
+
 	s.closeIdx()
 	if s.Decompressor == nil {
 		return nil