Output txid and serialized tx in transaction json serializer

[araspitzu]

For ease of use this PR improves the transaction serializer to output the transaction id along with the serialized transaction, previously we outputted only the serialized transaction.

[codecov-io]

Codecov Report

Merging #1033 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1033      +/-   ##
==========================================
- Coverage   80.58%   80.51%   -0.07%     
==========================================
  Files          98       98              
  Lines        7618     7618              
  Branches      299      299              
==========================================
- Hits         6139     6134       -5     
- Misses       1479     1484       +5

Impacted Files	Coverage Δ
...in/scala/fr/acinq/eclair/api/JsonSerializers.scala	96.96% <ø> (ø)	⬆️
...clair/blockchain/electrum/ElectrumClientPool.scala	75.26% <0%> (-7.53%)	⬇️
...cinq/eclair/blockchain/bitcoind/zmq/ZMQActor.scala	89.74% <0%> (-5.13%)	⬇️
.../acinq/eclair/blockchain/electrum/Blockchain.scala	39.68% <0%> (-1.59%)	⬇️
...nq/eclair/blockchain/electrum/ElectrumClient.scala	71.92% <0%> (ø)	⬆️
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	83.55% <0%> (+0.19%)	⬆️
...src/main/scala/fr/acinq/eclair/router/Router.scala	86.76% <0%> (+0.94%)	⬆️

[pm47]

I'm surprised this hasn't broken any existing API test.