Make Setup.datadir a val

[akumaigorodski]

This will allow plugins to know where the Eclair datadir is and look for their files in that dir. Fixes #1830

[codecov-commenter]

Codecov Report

Merging #1837 (56d9124) into master (dbecb28) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1837   +/-   ##
=======================================
  Coverage   89.38%   89.39%           
=======================================
  Files         145      145           
  Lines       10980    10980           
  Branches      442      442           
=======================================
+ Hits         9815     9816    +1     
+ Misses       1165     1164    -1     

Impacted Files	Coverage Δ
...ir-core/src/main/scala/fr/acinq/eclair/Setup.scala	80.35% <ø> (ø)
...cala/fr/acinq/eclair/payment/relay/NodeRelay.scala	96.09% <0.00%> (-1.57%)	⬇️
...main/scala/fr/acinq/eclair/io/PeerConnection.scala	84.68% <0.00%> (-0.43%)	⬇️
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	86.35% <0.00%> (+0.16%)	⬆️
...in/scala/fr/acinq/eclair/channel/TxPublisher.scala	98.21% <0.00%> (+1.19%)	⬆️

[t-bast]

Are you sure this changes anything?
It seems to me that the existing code already makes it a val.

[akumaigorodski]

Setup is not a case class where all constructor parameters are vals by default, without adding a val explicitly datadir is not visible outside of Setup.

[t-bast]

So it's for visibility, not immutability.
I thought it was to ensure that internally setup couldn't mutate that datadir.
For visibility that makes sense, LGTM.