Refactor channel transitions

[t-bast]

This PR contains changes from #2225 without the channel data type hierarchy changes.
It doesn't introduce a lot of functional changes, but they are quite sensitive: reviewers beware!
This is better reviewed commit-by-commit.

[codecov-commenter]

Codecov Report

Merging #2228 (eaa488b) into master (36ed788) will increase coverage by 0.02%.
The diff coverage is 80.64%.

@@            Coverage Diff             @@
##           master    #2228      +/-   ##
==========================================
+ Coverage   83.86%   83.88%   +0.02%     
==========================================
  Files         190      190              
  Lines       13967    13964       -3     
  Branches      585      578       -7     
==========================================
+ Hits        11713    11714       +1     
+ Misses       2254     2250       -4     

Impacted Files	Coverage Δ
...n/scala/fr/acinq/eclair/balance/BalanceActor.scala	10.16% <ø> (ø)
...ala/fr/acinq/eclair/balance/ChannelsListener.scala	93.10% <ø> (ø)
...n/scala/fr/acinq/eclair/balance/CheckBalance.scala	66.30% <ø> (ø)
...c/main/scala/fr/acinq/eclair/channel/Helpers.scala	94.97% <ø> (ø)
...q/eclair/channel/fsm/ChannelOpenSingleFunder.scala	90.74% <ø> (ø)
...a/fr/acinq/eclair/channel/fsm/CommonHandlers.scala	90.00% <ø> (ø)
.../acinq/eclair/db/migration/CompareChannelsDb.scala	0.00% <0.00%> (ø)
.../acinq/eclair/db/migration/MigrateChannelsDb.scala	0.00% <0.00%> (ø)
...-core/src/main/scala/fr/acinq/eclair/io/Peer.scala	90.41% <ø> (ø)
...rc/main/scala/fr/acinq/eclair/io/Switchboard.scala	85.41% <ø> (ø)
... and 17 more