Support paying or reimbursing fees from ROFL app pool

[kostko]

We should consider a new ROFL app fee policy where the per-ROFL app pool would cover the transaction fees. One important detail is handling DoS vectors for instance registration transactions as for first-time registrations one needs to make sure that verification fees have been paid before performing potentially expensive verification.

Possibilities include:

• Fees are reimbursed in case of successful registration. Note that this would still require some funds (e.g. from the endorsing node) to be available.
• First-time registration needs to generate a ZK proof of its validity which is cheaper to verify.