Better gas handling for `submit_reveal_aux`

[grarco]

At the moment, the client automatically detect whether the submission of a reveal_pk transaction is needed before submitting the intended transaction. The auxiliary reveal_pk transaction requires a gas limit and a gas payment and we are currently setting everything to the same values used for the intended transaction. This is not quite correct because:

• The gas limit might be insufficient for the reveal pk transaction
• The gas limit could be higher than the minimum required to reveal a public key leading to the gas payer spending too much on fees

We should manage the gas aspect of this ancillary tx better.

We can instead assume that the gas-price can be kept the same for both transactions as that define the priority that the signer is willing to give to the transaction which must be kept constant among the two to be meaningful.