Correctly set gossip sync_complete

[t-bast]

We are restoring the previous behavior of using the sync_complete field to signal the end of a channel_range_query sync.
The first step is to correctly set that field, before we can read it and interpret it to mark the end of sync.

See lightning/bolts#826 for more details.

[codecov-io]

Codecov Report

Merging #1668 (dfdb2b4) into master (ac054a2) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1668      +/-   ##
==========================================
- Coverage   85.95%   85.92%   -0.04%     
==========================================
  Files         151      151              
  Lines       11457    11460       +3     
  Branches      484      491       +7     
==========================================
- Hits         9848     9847       -1     
- Misses       1609     1613       +4     

Impacted Files	Coverage Δ
...e/src/main/scala/fr/acinq/eclair/router/Sync.scala	98.39% <100.00%> (+0.02%)	⬆️
...a/fr/acinq/eclair/wire/LightningMessageTypes.scala	86.36% <100.00%> (ø)
...cinq/eclair/blockchain/bitcoind/zmq/ZMQActor.scala	90.00% <0.00%> (-5.00%)	⬇️
...main/scala/fr/acinq/eclair/router/Validation.scala	90.76% <0.00%> (-1.54%)	⬇️
...nq/eclair/blockchain/electrum/ElectrumWallet.scala	80.25% <0.00%> (-0.26%)	⬇️
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	86.10% <0.00%> (ø)
.../acinq/eclair/blockchain/bitcoind/ZmqWatcher.scala	99.27% <0.00%> (+0.72%)	⬆️
...cala/fr/acinq/eclair/payment/relay/NodeRelay.scala	95.27% <0.00%> (+1.57%)	⬆️