Replace start and end block range with nonce in the DataCommitmentConfirms

[evan-forbes]

Currently we are using a block range for each data commitment. We should instead replace this with a nonce, as this is more in line with what the contracts expect. This will make relaying and slashing easier.

[rach-id]

I guess, if we go for a nonce, it is better if we separate:
MsgDataCommitment and MsgDataCommitmentConfirm, similar to what ValSet and ValSetConfirm do, with the prior containing a nonce and the latter reference to it.
What do you think ?

[evan-forbes]

yeah, that makes sense! Following the pattern is a good idea. We will likely also follow the ValsetRequest pattern too in order to accomodate celestiaorg/blobstream-contracts#54 and #332

[evan-forbes]

can close since we switched to universal nonces