Configure dust in flight threshold

[t-bast]

This configuration field lets node operators decide on the amount of dust htlcs that can be in-flight in each channel.
In case the channel is force-closed, up to this amount may be lost in miner fees.

When sending and receiving htlcs, we check whether they would overflow our configured dust exposure, and fail them instantly if they do.

A large update_fee may overflow our dust exposure by removing from the commit tx htlcs that were previously untrimmed.
Node operators can choose to automatically force-close when that happens, to avoid risking losing large dust amounts to miner fees.
This is disabled by default.

[codecov-commenter]

Codecov Report

Merging #1985 (ca5acf3) into master (d6b46ae) will increase coverage by 0.03%.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master    #1985      +/-   ##
==========================================
+ Coverage   87.60%   87.63%   +0.03%     
==========================================
  Files         159      160       +1     
  Lines       12445    12531      +86     
  Branches      529      537       +8     
==========================================
+ Hits        10902    10982      +80     
- Misses       1543     1549       +6     

Impacted Files	Coverage Δ
.../fr/acinq/eclair/blockchain/fee/FeeEstimator.scala	100.00% <ø> (ø)
.../fr/acinq/eclair/transactions/CommitmentSpec.scala	83.72% <ø> (ø)
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	88.13% <75.00%> (-0.20%)	⬇️
...n/scala/fr/acinq/eclair/channel/DustExposure.scala	96.96% <96.96%> (ø)
...in/scala/fr/acinq/eclair/channel/Commitments.scala	94.51% <97.95%> (+0.61%)	⬆️
...re/src/main/scala/fr/acinq/eclair/NodeParams.scala	91.85% <100.00%> (+0.22%)	⬆️
...la/fr/acinq/eclair/transactions/Transactions.scala	96.80% <100.00%> (+0.01%)	⬆️
.../acinq/eclair/blockchain/bitcoind/ZmqWatcher.scala	88.13% <0.00%> (-1.87%)	⬇️
...clair/channel/publish/ReplaceableTxPublisher.scala	86.20% <0.00%> (-1.15%)	⬇️
... and 2 more