fullnode does not detect fork after rollback until new block is created

[srene]

fork could be detected by fork loop before new block arrives. the problem is fork loop exits when there is an instruction and therefore it will not detect the fork and freeze when reaching the fork height, unless new block is created and freezes from sync loop. not a big issue but could be improved. fork loop could not exit when there is an instruction file but just not check for fork while there is whithout finishing the loop.

[srene]

fixed by #1261