Analyze mempool/estimatesmartfee data and re-calibrate normal feerate estimate parameters

[0e319b6]

See #4113 .

Previously in IRC @rustyrussell and I agreed it would be useful to have some data on how the mempool and estimatesmartfee results change over time in order to figure out the best way to estimate fees.

I'll be happy to update the value of "normal" too over, depending on what our best guess is from your observations.

The observation data can be found here.

[hosiawak]

Please see my comments about issues with current bitcoin core feerate estimates here

tl;dr Pluggable feerate estimators would be great so we don't have to rely on core's "very interesting" estimates.