Removed cached codec

[pm47]

It turns out that performance gains of the cached codec are not that
great, and they come at a cost of significant pressure on the GC. In
other words: premature optimization.

When removed, the heap usage becomes very stable, which is much better
than hypothetical performance gains.

Fixes #1031.

[codecov-io]

Codecov Report

Merging #1036 into master will decrease coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1036      +/-   ##
==========================================
- Coverage   80.59%   80.51%   -0.08%     
==========================================
  Files          98       98              
  Lines        7631     7622       -9     
  Branches      309      309              
==========================================
- Hits         6150     6137      -13     
- Misses       1481     1485       +4

Impacted Files	Coverage Δ
.../fr/acinq/eclair/wire/LightningMessageCodecs.scala	99.13% <ø> (+0.73%)	⬆️
.../main/scala/fr/acinq/eclair/io/Authenticator.scala	92% <100%> (ø)	⬆️
...src/main/scala/fr/acinq/eclair/router/Router.scala	85.81% <0%> (-1.66%)	⬇️
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	83.46% <0%> (-0.2%)	⬇️
...nq/eclair/blockchain/electrum/ElectrumClient.scala	72.65% <0%> (+0.37%)	⬆️
...clair/blockchain/electrum/ElectrumClientPool.scala	78.49% <0%> (+3.22%)	⬆️