rx (fix): Avoid concurrent modification of RxVar subscribers

[xerial]

• Fixes OkHttp Dispatcher: java.util.ConcurrentModificationException: mutation occurred during iteration  #3109
• Fix deprecated method usage
• rx (fix): Sychrnoize variable update

[codecov]

Codecov Report

Merging #3113 (8d727da) into master (9c0a70a) will decrease coverage by 0.01%.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3113      +/-   ##
==========================================
- Coverage   82.85%   82.84%   -0.01%     
==========================================
  Files         350      350              
  Lines       14718    14723       +5     
  Branches     2420     2447      +27     
==========================================
+ Hits        12195    12198       +3     
- Misses       2523     2525       +2     

Files Changed	Coverage Δ
...ala/wvlet/airframe/http/okhttp/OkHttpChannel.scala	91.66% <100.00%> (ø)
...ain/scala/wvlet/airframe/http/okhttp/package.scala	72.00% <100.00%> (ø)
...me-rx/src/main/scala/wvlet/airframe/rx/RxVar.scala	86.66% <100.00%> (+2.66%)	⬆️

... and 1 file with indirect coverage changes

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