grpc: Use ForkJoinPool with nCPU x 2 max thread by default

[xerial]

No description provided.

[codecov]

Codecov Report

Merging #1557 (961232f) into master (7b360c7) will increase coverage by 0.03%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master    #1557      +/-   ##
==========================================
+ Coverage   82.89%   82.92%   +0.03%     
==========================================
  Files         306      306              
  Lines       12115    12126      +11     
  Branches      755      736      -19     
==========================================
+ Hits        10043    10056      +13     
+ Misses       2072     2070       -2     

Impacted Files	Coverage Δ
...in/scala/wvlet/airframe/http/grpc/GrpcServer.scala	67.74% <90.90%> (+12.74%)	⬆️
...rx/src/main/scala/wvlet/airframe/rx/RxRunner.scala	92.82% <0.00%> (+0.89%)	⬆️
...og/shared/src/main/scala/wvlet/log/LogFormat.scala	92.00% <0.00%> (+2.00%)	⬆️

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