SCORU/Node: fix issue with level information during reorganisation

Context

This MR fixes an issue with the way the rollup node associates levels to block hashes when there is a reorganisation involving a block that was already seen.

level:  1     2     3 

chain:  A --> B --> C
         \--> B'--> C'   (reorg 1)
         \--> B --> C''  (reorg 2) 

In the second reorg, the level 2 would stay associated to B' while it should have been reset to B.