Allow deposits with only 1 commit

[L-as]

Right now you have to submit the action, commit, process it, commit again for it to be finalised (modulo L1 rollbacks).

We should be able to do this with only one commit by processing it tentatively and checking it's correct commit-time.

[L-as]

Possible solution:

Allow more than one inner step per outer step. For each outer deposit submission, sequencer adds inner step that updates all_deposits.

[MartinOndejka]

Related #112

[L-as]

Made less important by the fact that even if we supported this now, rollbacks would cause wasted work.