Check for message inclusion before committing to a block

[evan-forbes]

Since we will be using ABCI++ soon, it should be much easier to implement a message inclusion check that can be performed when a validator processes a proposal block. This issue has been described in more depth previously in celestiaorg/celestia-core#520 and #1006. We also have a pending ADR celestiaorg/celestia-core#559

This check should ensure that

• each valid PayForMessage has a corresponding message with the correct commitment
• that there are no unaccounted for messages included in the square

blocked by #381 (might be able to be done in the same PR since they are so related)

blocked by celestiaorg/celestia-core#204