Fix availableForSend/Receive

[t-bast]

There was a rounding issue with the availableForSend/Receive calculation.
Because CommitTx fee and Htlc fee were computed separately,
but each was individually rounded down to Satoshis, we could
end up with an off-by-one error.

This resulted in an incapacity to send/receive the maximum amount available.

We now allow computing fees in msat, which removes rounding issues.

[t-bast]

Great, it looks like the fuzz test has found something :)
I'm investigating that (it may be an issue with the test itself).

EDIT: it had run into this known issue: lightning/bolts#728
We can't really fix it for now, so we just make sure the test doesn't get into that state.

[codecov-io]

Codecov Report

Merging #1293 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1293      +/-   ##
==========================================
+ Coverage   77.18%   77.35%   +0.16%     
==========================================
  Files         143      143              
  Lines        9976     9978       +2     
  Branches      408      407       -1     
==========================================
+ Hits         7700     7718      +18     
+ Misses       2276     2260      -16

Impacted Files	Coverage Δ
...in/scala/fr/acinq/eclair/channel/Commitments.scala	90.32% <100%> (ø)	⬆️
...la/fr/acinq/eclair/transactions/Transactions.scala	96.77% <100%> (+0.02%)	⬆️
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	83.19% <0%> (-0.37%)	⬇️
...nq/eclair/blockchain/electrum/ElectrumWallet.scala	81% <0%> (+0.25%)	⬆️
...nq/eclair/blockchain/electrum/ElectrumClient.scala	73.64% <0%> (+0.36%)	⬆️
...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%)	⬆️
...c/main/scala/fr/acinq/eclair/crypto/ShaChain.scala	94.87% <0%> (+2.56%)	⬆️
...cinq/eclair/blockchain/bitcoind/zmq/ZMQActor.scala	94.87% <0%> (+5.12%)	⬆️
...clair/blockchain/electrum/ElectrumClientPool.scala	84.94% <0%> (+10.75%)	⬆️