Clarify ChannelsDb htlc info method

[t-bast]

The old name (addOrUpdateHtlcInfo) was misleading.
This method only adds, and can even handle adding duplicate lines.

Closes #1233

[codecov-io]

Codecov Report

Merging #1291 into master will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #1291     +/-   ##
=========================================
+ Coverage   77.18%   77.28%   +0.1%     
=========================================
  Files         143      143             
  Lines        9976     9976             
  Branches      408      408             
=========================================
+ Hits         7700     7710     +10     
+ Misses       2276     2266     -10

Impacted Files	Coverage Δ
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	83.56% <100%> (ø)	⬆️
...a/fr/acinq/eclair/db/sqlite/SqliteChannelsDb.scala	95.91% <100%> (ø)	⬆️
...nq/eclair/blockchain/electrum/ElectrumWallet.scala	81% <0%> (+0.25%)	⬆️
...src/main/scala/fr/acinq/eclair/router/Router.scala	92.97% <0%> (+0.68%)	⬆️
...q/eclair/blockchain/electrum/ElectrumWatcher.scala	52.13% <0%> (+0.85%)	⬆️
...clair/blockchain/electrum/ElectrumClientPool.scala	78.49% <0%> (+4.3%)	⬆️

[pm47]

Good catch @btcontract