Use long id instead of short id in ChannelRelay

[pm47]

This is a preparation for lightning/bolts#910.

The relaying logic is now based on the channelId instead of
shortChannelId.

We used to consider shortChannelId as a unique identifier for the
channel, but it's not true anymore: there will be multiple aliases per
channel.

[t-bast]

Looks mostly good to me 👍

[codecov-commenter]

Codecov Report

Merging #2221 (848062f) into master (95213bc) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2221      +/-   ##
==========================================
- Coverage   84.35%   84.30%   -0.05%     
==========================================
  Files         194      194              
  Lines       14393    14411      +18     
  Branches      612      568      -44     
==========================================
+ Hits        12141    12149       +8     
- Misses       2252     2262      +10     

Impacted Files	Coverage Δ
...a/fr/acinq/eclair/payment/relay/ChannelRelay.scala	98.03% <100.00%> (-0.02%)	⬇️
...fr/acinq/eclair/payment/relay/ChannelRelayer.scala	100.00% <100.00%> (ø)
.../scala/fr/acinq/eclair/payment/relay/Relayer.scala	90.90% <100.00%> (+0.21%)	⬆️
...cala/fr/acinq/eclair/payment/relay/NodeRelay.scala	95.93% <0.00%> (-1.63%)	⬇️
...main/scala/fr/acinq/eclair/router/Validation.scala	90.87% <0.00%> (-1.58%)	⬇️
...clair/channel/publish/ReplaceableTxPublisher.scala	72.84% <0.00%> (-1.33%)	⬇️
...in/scala/fr/acinq/eclair/channel/fsm/Channel.scala	88.27% <0.00%> (-0.12%)	⬇️
...main/scala/fr/acinq/eclair/io/PeerConnection.scala	85.98% <0.00%> (ø)
...cinq/eclair/channel/publish/MempoolTxMonitor.scala	88.88% <0.00%> (ø)
...ire/internal/channel/version0/ChannelCodecs0.scala	96.15% <0.00%> (ø)
... and 3 more