Add payCommitTxFees flag to LocalParams

[t-bast]

The channel initiator traditionnally pays the commit tx fees, but we may want to override that when providing services to wallet users. We thus split the current isInitiator flag into two flags:

• isChannelOpener
• payCommitTxFees

We always set payCommitTxFees to the same value as isChannelOpener. Custom feature bits may override that behavior if necessary.

Note that backwards compatibity is preserved since our previous bool8 codec encodes true as 0xff and false as 0x00.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.42105% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 86.01%. Comparing base (c8184b3) to head (d3fbd8e).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2845      +/-   ##
==========================================
+ Coverage   85.96%   86.01%   +0.04%     
==========================================
  Files         219      219              
  Lines       18441    18441              
  Branches      762      790      +28     
==========================================
+ Hits        15853    15862       +9     
+ Misses       2588     2579       -9     

Files	Coverage Δ
...in/scala/fr/acinq/eclair/channel/ChannelData.scala	100.00% <100.00%> (ø)
.../scala/fr/acinq/eclair/channel/ChannelEvents.scala	100.00% <ø> (ø)
...c/main/scala/fr/acinq/eclair/channel/Helpers.scala	94.53% <100.00%> (ø)
...inq/eclair/channel/fsm/ChannelOpenDualFunded.scala	87.00% <100.00%> (ø)
...q/eclair/channel/fsm/ChannelOpenSingleFunded.scala	93.52% <100.00%> (ø)
...la/fr/acinq/eclair/channel/fsm/ErrorHandlers.scala	83.55% <100.00%> (ø)
...q/eclair/crypto/keymanager/ChannelKeyManager.scala	87.50% <ø> (ø)
...main/scala/fr/acinq/eclair/db/DbEventHandler.scala	93.58% <100.00%> (ø)
...c/main/scala/fr/acinq/eclair/db/pg/PgAuditDb.scala	99.69% <100.00%> (ø)
...cala/fr/acinq/eclair/db/sqlite/SqliteAuditDb.scala	99.67% <100.00%> (ø)
... and 13 more

... and 4 files with indirect coverage changes

[pm47]

IIUC we:

• use isChannelOpened:
  • as InteractiveTxParams.isInitiator
  • in the switch-to-zeroconf logic
  • to compute obscuredCommitTxNumber()
  • in the rate limit logic
• use payCommitTxFees:
  • for commitment tx fees
  • for closing tx fees
  • to decide whether to send update_fee

That LGTM

[t-bast]

It's also worth noting that this PR is perfectly safe to integrate: we introduce the paysCommitTxFees boolean flag, but there is no way to set it to something else than isChannelOpener (as this should only be set for on-the-fly funding).