Certificate submission process improvement

[albertog78]

To have the possibility of having for example different set of submitters depending on mainchain block height would be needed to have the submission event to be triggered based on mainchain block appearance (not on mainchain block reference inclusion because this could lead to some delays in submission). This applies to all the mainchain blocks successors of the mainchain block that identifies the end of the epoch. Instead for the mainchain block of the eond of the epoch we should wait for its inclusion in a sidechain block. Only after that we should start monitor mainchain block appear. This is necessary because in the final implementation to produce the certificate and the proof we will need to know the status of the sidechain at the end of the epoch and only after that start submitting the certificate given the rules described in the beginning.