RFC: simplifications and suggestions on open/accept limits.

In an email thread between Gert-Jaap Glasbergen and Christian Decker, the latter noted that dust rules prevented them from creating outputs smaller than 546 satoshis on Bitcoin, which means fees and transferred values always have to be multiples of that dust limit. However, Decker believes that the 546 satoshis is no longer a tiny amount as compared to current minimum fee and value transferred, and they will have to deal with values that aren't representable or enforceable on-chain anyway. While Gert-Jaap found this illogical, Decker explains that it doesn't make sense to enforce mandatory limitations on base layer enforcement but rather provide options in the protocol so that an implementation can choose what security level it prefers.