Rollup node/Store: use predecessor block hash in messages header

Context

This MR changes the information associated to messages in the store of the rollup node. The block hash (of the inbox where the messages appear) is replaced with its predecessor. This information is enough to compute the inbox proofs and is not dependant on reorganizations.

The problem solved by this MR is that in case of reorg and a later GC the information about a block hash contained in the header of the messages store can disappear and prevent the successful playing of refutation games.

Manually testing the MR

Rollup node should still be able to win refutation games for inbox proofs. (See CI.)