better access to the fee estimation

Currently, if we want to estimate a fee in Nakamoto, we need to require a block, and Nakamoto keeps track of the fee with an estimation logic. In this PR (https://github.com/cloudhead/nakamoto/pull/139), I added a naive fee estimation, but some work needs to be done, such as:

- Not storing all the estimated fees for all blocks in memory: after a certain limit, we should cut the hash map.
- Being able to persist the calculated fee: We should be able to store the calculated fee to enable quick responses across restarts. See https://github.com/coffee-tools/folgore/issues/20 for a use case.