Init_chain should return the genesis root hash and have post genesis hooks

[theochap]

Context
Currently, the init_chain function does not return the initial root hash after genesis. This forces us to use hacks and process an empty blob such as in the demo-rollup. Moreover, the rollup needs to be able to get the initial state hash to properly initialize the chain state.

Solution

• Return the initial root hash after genesis in the init_chain function.
  • Done as part of Chain state module #598
• Hook on the exit of the init_chain function to be able to store the initial root hash. That would be some sort of genesis_hook.

[citizen-stig]

@theochap could you please confirm if this issue is still have left things to do?

[citizen-stig]

Already one!