Allow plugins to force reconnect

[akumaigorodski]

Discussed at https://gitter.im/ACINQ/developers?at=5fa1289c2a3544071500e1b3, this will allow message-enabled plugins to keep a connection open to a given node even if there are no channels left.

[codecov-io]

Codecov Report

Merging #1594 (207fade) into master (e5db314) will increase coverage by 0.05%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #1594      +/-   ##
==========================================
+ Coverage   87.31%   87.36%   +0.05%     
==========================================
  Files         146      146              
  Lines       11056    11061       +5     
  Branches      471      466       -5     
==========================================
+ Hits         9653     9663      +10     
+ Misses       1403     1398       -5     

Impacted Files	Coverage Δ
.../src/main/scala/fr/acinq/eclair/PluginParams.scala	0.00% <ø> (ø)
...main/scala/fr/acinq/eclair/io/PeerConnection.scala	82.12% <0.00%> (+0.42%)	⬆️
...re/src/main/scala/fr/acinq/eclair/NodeParams.scala	91.86% <100.00%> (+0.14%)	⬆️
...in/scala/fr/acinq/eclair/io/ReconnectionTask.scala	93.02% <100.00%> (ø)
...nq/eclair/blockchain/electrum/ElectrumWallet.scala	80.25% <0.00%> (-0.26%)	⬇️
...in/scala/fr/acinq/eclair/channel/Commitments.scala	93.91% <0.00%> (ø)
...cala/fr/acinq/eclair/router/RouteCalculation.scala	99.32% <0.00%> (+<0.01%)	⬆️
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	86.06% <0.00%> (+0.08%)	⬆️
...cala/fr/acinq/eclair/payment/relay/NodeRelay.scala	95.27% <0.00%> (+1.57%)	⬆️
... and 1 more