Don't rebroadcast channel updates from `update_fail_htlc`

[t-bast]

When we receive a channel_update in update_fail_htlc, we should take it into account to exclude channels or correctly retry with updated fees, but we shouldn't apply it to our routing table.

If we did, that could be exploited to deanonymize our payments. It shouldn't be necessary anyway, as honest nodes should broadcast those channel_updates publicly, so we would receive them through the normal gossip mechanism.

Fixes #2767

[codecov-commenter]

Codecov Report

Merging #2775 (fe969e6) into master (5fa7d4b) will decrease coverage by 0.02%. Report is 1 commits behind head on master. The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master    #2775      +/-   ##
==========================================
- Coverage   85.83%   85.82%   -0.02%     
==========================================
  Files         216      216              
  Lines       18129    18128       -1     
  Branches      775      773       -2     
==========================================
- Hits        15561    15558       -3     
- Misses       2568     2570       +2     

Files	Coverage Δ
...r/acinq/eclair/payment/send/PaymentLifecycle.scala	86.84% <ø> (-0.07%)	:arrow_down:

... and 8 files with indirect coverage changes