Add a util method to warm up EventLoopGroup

[kezhenxu94]

Motivation:

Netty's event loop threads are spawned lazily, i.e. they are created
on the first task is scheduled on. A user sometimes wants to ensure
the event loop threads are started when the server starts up.

Modifications:

• Add EventLoopGroups.warmUp()

Result:

• Closes EventLoops.warmUp(EventLoopGroup) #3363

[trustin]

[codecov]

Codecov Report

Merging #3610 (22ca9d8) into master (c47d84e) will decrease coverage by 0.05%.
The diff coverage is 25.00%.

@@             Coverage Diff              @@
##             master    #3610      +/-   ##
============================================
- Coverage     73.89%   73.83%   -0.06%     
- Complexity    14367    14378      +11     
============================================
  Files          1260     1263       +3     
  Lines         54819    54878      +59     
  Branches       7021     7021              
============================================
+ Hits          40507    40521      +14     
- Misses        10732    10794      +62     
+ Partials       3580     3563      -17     

Impacted Files	Coverage Δ
.../linecorp/armeria/common/util/EventLoopGroups.java	42.85% <25.00%> (-2.31%)	⬇️
...rp/armeria/server/tomcat/ManagedTomcatService.java	50.84% <0.00%> (-22.04%)	⬇️
...nternal/server/tomcat/Tomcat90ProtocolHandler.java	50.00% <0.00%> (-15.39%)	⬇️
...rp/armeria/server/tomcat/TomcatServiceBuilder.java	36.36% <0.00%> (-10.91%)	⬇️
...om/linecorp/armeria/client/HttpSessionHandler.java	73.14% <0.00%> (-8.58%)	⬇️
.../com/linecorp/armeria/server/docs/ServiceInfo.java	78.94% <0.00%> (-1.76%)	⬇️
.../com/linecorp/armeria/server/RoutingPredicate.java	70.96% <0.00%> (-1.62%)	⬇️
...m/linecorp/armeria/common/ClientCookieDecoder.java	91.66% <0.00%> (-1.05%)	⬇️
...a/com/linecorp/armeria/client/HttpChannelPool.java	78.41% <0.00%> (-0.28%)	⬇️
.../java/com/linecorp/armeria/common/CommonPools.java	100.00% <0.00%> (ø)
... and 16 more

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[ikhoon]

Thanks! @kezhenxu94

[minwoox]

Thanks!