Add fee credit feature for on-the-fly funding

[t-bast]

We add an optional feature that lets on-the-fly funding clients accept payments that are too small to pay the fees for an on-the-fly funding. When that happens, the payment amount is added as "fee credit" without performing an on-chain operation. Once enough fee credit has been obtained, we can initiate an on-chain operation to create a channel or a splice by paying part of the fees from our fee credit.

This feature makes more efficient use of on-chain transactions by trusting that our peer will honor our fee credit in the future. The fee credit takes precedence over other ways of paying the fees (from our channel balance or future HTLCs), which guarantees that the fee credit eventually converges to 0.

This PR builds on top of #649.