Allow using outgoing channel id for onion messages

[thomash-acinq]

Allow using a short channel id instead of a public key to designate the next node a message should be relayed to.

[codecov-commenter]

Codecov Report

Merging #2762 (574aedf) into master (9ca9227) will increase coverage by 0.00%. Report is 2 commits behind head on master. The diff coverage is 97.61%.

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@@           Coverage Diff           @@
##           master    #2762   +/-   ##
=======================================
  Coverage   85.84%   85.84%           
=======================================
  Files         216      216           
  Lines       18102    18111    +9     
  Branches      762      740   -22     
=======================================
+ Hits        15540    15548    +8     
- Misses       2562     2563    +1     

Files	Coverage Δ
...re/src/main/scala/fr/acinq/eclair/NodeParams.scala	93.39% <100.00%> (-0.03%)	:arrow_down:
...ir-core/src/main/scala/fr/acinq/eclair/Setup.scala	75.14% <100.00%> (ø)
.../main/scala/fr/acinq/eclair/channel/Register.scala	88.57% <100.00%> (+0.33%)	:arrow_up:
...-core/src/main/scala/fr/acinq/eclair/io/Peer.scala	87.71% <100.00%> (+0.10%)	:arrow_up:
...rc/main/scala/fr/acinq/eclair/io/Switchboard.scala	85.71% <100.00%> (-0.50%)	:arrow_down:
.../scala/fr/acinq/eclair/message/OnionMessages.scala	79.71% <100.00%> (+2.29%)	:arrow_up:
...c/main/scala/fr/acinq/eclair/message/Postman.scala	95.65% <100.00%> (+0.06%)	:arrow_up:
...cinq/eclair/remote/EclairInternalsSerializer.scala	97.82% <100.00%> (ø)
...scala/fr/acinq/eclair/router/BalanceEstimate.scala	98.93% <100.00%> (ø)
.../src/main/scala/fr/acinq/eclair/router/Graph.scala	97.25% <100.00%> (-0.09%)	:arrow_down:
... and 6 more

... and 7 files with indirect coverage changes