Better randomization of reconnection delay

[pm47]

Randomization is necessary, otherwise if two peers attempt to reconnect
to each other in a synchronized fashion, they will enter in a
disconnect-reconnect loop.

We already had randomization for the initial reconnection attempt, but
further reconnection attempts were using a deterministic schedule
following an exponential backoff curve.

Fixes #1238.

[araspitzu]

LGTM

[codecov-io]

Codecov Report

Merging #1250 into master will increase coverage by 0.03%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #1250      +/-   ##
==========================================
+ Coverage   76.66%   76.69%   +0.03%     
==========================================
  Files         140      140              
  Lines        9702     9703       +1     
  Branches      376      380       +4     
==========================================
+ Hits         7438     7442       +4     
+ Misses       2264     2261       -3

Impacted Files	Coverage Δ
...-core/src/main/scala/fr/acinq/eclair/io/Peer.scala	75.59% <50%> (+0.07%)	⬆️
...src/main/scala/fr/acinq/eclair/router/Router.scala	92% <0%> (-0.72%)	⬇️
...nq/eclair/blockchain/electrum/ElectrumWallet.scala	81% <0%> (+0.25%)	⬆️
...q/eclair/blockchain/electrum/ElectrumWatcher.scala	51.72% <0%> (+1.72%)	⬆️
...clair/blockchain/electrum/ElectrumClientPool.scala	78.49% <0%> (+4.3%)	⬆️