Refactor: move channel relaying to a ChannelRelayer actor.

[t-bast]

The relayer is the top-level orchestrator that forwards to a payment handler, channel relayer or (coming) node relayer.

I want to get this out of the way before focusing on the NodeRelayer where most of the work for Trampoline will happen (mainly because git doesn't handle that well files that are moved across merge/rebase and I'm afraid I'll end up wasting time on that).

For some reason git doesn't mark relay/Relayer.scala as a move with edits, it instead marks it as a delete and an add, which may make it harder to review...

[codecov-io]

Codecov Report

Merging #1215 into master will decrease coverage by 0.02%.
The diff coverage is 89.61%.

@@            Coverage Diff             @@
##           master    #1215      +/-   ##
==========================================
- Coverage   76.32%   76.29%   -0.03%     
==========================================
  Files         139      140       +1     
  Lines        9474     9484      +10     
  Branches      395      401       +6     
==========================================
+ Hits         7231     7236       +5     
- Misses       2243     2248       +5

Impacted Files	Coverage Δ
...ir-core/src/main/scala/fr/acinq/eclair/Setup.scala	73.09% <ø> (ø)	⬆️
.../fr/acinq/eclair/payment/relay/CommandBuffer.scala	100% <ø> (ø)
...r-core/src/main/scala/fr/acinq/eclair/Eclair.scala	54.43% <ø> (ø)	⬆️
...clair/payment/send/MultiPartPaymentLifecycle.scala	97.88% <ø> (ø)	⬆️
...rc/main/scala/fr/acinq/eclair/io/Switchboard.scala	84.21% <ø> (ø)	⬆️
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	83.37% <ø> (-0.37%)	⬇️
...in/scala/fr/acinq/eclair/channel/Commitments.scala	89.23% <ø> (ø)	⬆️
...ain/scala/fr/acinq/eclair/wire/ChannelCodecs.scala	100% <100%> (ø)	⬆️
.../scala/fr/acinq/eclair/payment/relay/Relayer.scala	84% <84%> (ø)
...fr/acinq/eclair/payment/relay/ChannelRelayer.scala	94.73% <94.73%> (ø)
... and 4 more