Fixed race condition in payment handler init

[pm47]

When an actor sends a message to itself as part of its class definition,
there is no guarantee that this message will be processed first. Relying
on that to set the default payment handler is problematic and causes
race conditions in tests.

[codecov-io]

Codecov Report

Merging #1208 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1208      +/-   ##
==========================================
+ Coverage   76.17%   76.19%   +0.02%     
==========================================
  Files         138      138              
  Lines        9385     9385              
  Branches      372      367       -5     
==========================================
+ Hits         7149     7151       +2     
+ Misses       2236     2234       -2

Impacted Files	Coverage Δ
.../acinq/eclair/payment/receive/PaymentHandler.scala	100% <100%> (ø)	⬆️
...nq/eclair/blockchain/electrum/ElectrumWallet.scala	81.25% <0%> (+0.25%)	⬆️
...-core/src/main/scala/fr/acinq/eclair/io/Peer.scala	75.44% <0%> (+0.29%)	⬆️