FB 1/N: fee reserve estimates

[darosior]

The first milestone of the fee-bumping series. Tracking the historical (or not) estimates we'll need, keeping them up-to-date from one block to another efficiently, handling errors from our fee estimator or lack of data availability (eg the sim falls back to using historical block fees in this case).

[darosior]

Keep in mind we'll need to reproduce it in revaultd with much lower availability assumptions