Improve CommandSend type

[t-bast]

Add type with upper bound to make asInstanceOf unnecessary.
Split HasHtlcId from Command: they are orthogonal traits.

[codecov-io]

Codecov Report

Merging #1260 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1260      +/-   ##
==========================================
+ Coverage   77.03%   77.15%   +0.12%     
==========================================
  Files         142      142              
  Lines        9880     9880              
  Branches      402      402              
==========================================
+ Hits         7611     7623      +12     
+ Misses       2269     2257      -12

Impacted Files	Coverage Δ
.../fr/acinq/eclair/payment/relay/CommandBuffer.scala	100% <ø> (ø)	⬆️
.../acinq/eclair/db/sqlite/SqlitePendingRelayDb.scala	100% <ø> (ø)	⬆️
...n/scala/fr/acinq/eclair/channel/ChannelTypes.scala	90% <ø> (ø)	⬆️
...ain/scala/fr/acinq/eclair/wire/CommandCodecs.scala	100% <100%> (ø)	⬆️
...q/eclair/blockchain/electrum/ElectrumWatcher.scala	51.72% <0%> (-0.87%)	⬇️
...nq/eclair/blockchain/electrum/ElectrumClient.scala	73.28% <0%> (-0.37%)	⬇️
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	83.8% <0%> (+0.09%)	⬆️
...nq/eclair/blockchain/electrum/ElectrumWallet.scala	81% <0%> (+0.25%)	⬆️
...src/main/scala/fr/acinq/eclair/router/Router.scala	92.71% <0%> (+0.71%)	⬆️
...cala/fr/acinq/eclair/crypto/TransportHandler.scala	92.3% <0%> (+1.18%)	⬆️
... and 2 more

[pm47]

This is so much better.