Add private flag to channel updates

[t-bast]

Add a message flag to channel update to specify that an update is private and should not be rebroadcast to other nodes.
We log an error if a private channel update gets into the rebroadcast list.

I also tried strongly typing channel updates, dividing them into PrivateChannelUpdate and PublicChannelUpdate to get compile-time guarantees that private channel updates wouldn't leak where they're not supposed to, but unfortunately I hit some roadblocks...the main issue with this approach is that we would need Router.PublicChannel and Router.PrivateChannel to map to these two types of channel updates, but that's not how we use them internally: when a public channel isn't confirmed yet, we store it as a private channel which messes things up. Ideally we would need to rework this to clearly separate channels that will be public once confirmed from channels that will stay private, but that's quite a lot of work for not much benefits, so I'm leaving it aside for now.

cln has already merged support for this on their master branch, and I have successfully tested interop with them.

See lightning/bolts#999

[codecov-commenter]

Codecov Report

Merging #2362 (3f8a4eb) into master (1b36697) will increase coverage by 0.11%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master    #2362      +/-   ##
==========================================
+ Coverage   84.82%   84.94%   +0.11%     
==========================================
  Files         198      198              
  Lines       15761    15812      +51     
  Branches      655      711      +56     
==========================================
+ Hits        13369    13431      +62     
+ Misses       2392     2381      -11     

Impacted Files	Coverage Δ
...main/scala/fr/acinq/eclair/io/PeerConnection.scala	85.76% <50.00%> (-1.00%)	⬇️
...in/scala/fr/acinq/eclair/channel/fsm/Channel.scala	86.16% <100.00%> (-0.12%)	⬇️
...inq/eclair/channel/fsm/CommonFundingHandlers.scala	91.66% <100.00%> (ø)
...n/scala/fr/acinq/eclair/router/Announcements.scala	100.00% <100.00%> (ø)
...e/src/main/scala/fr/acinq/eclair/router/Sync.scala	98.39% <100.00%> (ø)
...fr/acinq/eclair/wire/protocol/FailureMessage.scala	83.09% <100.00%> (+0.24%)	⬆️
.../eclair/wire/protocol/LightningMessageCodecs.scala	100.00% <100.00%> (ø)
...q/eclair/wire/protocol/LightningMessageTypes.scala	98.36% <100.00%> (+0.08%)	⬆️
...cala/fr/acinq/eclair/payment/relay/NodeRelay.scala	96.55% <0.00%> (-1.01%)	⬇️
... and 33 more