Move fee provider conf

[pm47]

Took the opportunity to improve the naming as well.

I also considered putting them under a providers section, WDYT?

on-chain-fees {
  providers {
    min-feerate = 1 // minimum feerate in satoshis per byte
    smoothing-window = 6 // 1 = no smoothing
    timeout = 5 seconds // max time we'll wait for answers from a fee provider before we fallback to the next one
  }
...
}

[t-bast]

LGTM.

I don't think we need to add a providers section, since we'll eventually get rid of providers like bitgo and earndotcom and instead only use bitcoin core.