More fine grained support for fee diff errors

[t-bast]

When there is a mismatch between the feerate of a channel and the feerate we get from our estimator, we may want to force-close because that could be exploited by our peer to steal HTLCs. But that's only the case if the feerate is too low, not if it's too high. We previously force-closed in both cases, whereas we only need to do it when the feerate is too low.

This should avoid some unnecessary force-close that we've observed and are due to buggy fee estimators (fee estimation is hard!), or to peers who simply do some smoothing and slightly delay lowering the feerate of our channels.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.96%. Comparing base (e66e6d2) to head (5c9a5e8).
Report is 12 commits behind head on master.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2815      +/-   ##
==========================================
+ Coverage   85.86%   85.96%   +0.09%     
==========================================
  Files         216      219       +3     
  Lines       18228    18453     +225     
  Branches      772      795      +23     
==========================================
+ Hits        15652    15863     +211     
- Misses       2576     2590      +14     

Files	Coverage Δ
...r/acinq/eclair/blockchain/fee/OnChainFeeConf.scala	95.65% <100.00%> (+0.65%)	⬆️
...in/scala/fr/acinq/eclair/channel/Commitments.scala	96.83% <100.00%> (+0.01%)	⬆️
...in/scala/fr/acinq/eclair/channel/fsm/Channel.scala	85.49% <100.00%> (-0.31%)	⬇️

... and 37 files with indirect coverage changes