WIP: ThreadGroup multi-tasking

[vlsi]

Description

See https://lists.apache.org/thread.html/df0349a01889bec3a202f25dfdcf1ecd7fc9da535a48799576aff4bb@%3Cdev.jmeter.apache.org%3E

Motivation and Context

JMeter should not block threads on long pauses. That would enable testing with big "think times" with little amount of threads (==save RAM).

Java's Loom

https://mail.openjdk.java.net/pipermail/loom-dev/2019-December/000931.html

Checklist:

• try adding Kotlin
• Implement multi-tasking

[codecov-io]

Codecov Report

Merging #540 into master will increase coverage by 0.53%.
The diff coverage is 51.33%.

@@             Coverage Diff              @@
##             master     #540      +/-   ##
============================================
+ Coverage     55.46%   55.99%   +0.53%     
- Complexity     9964     9975      +11     
============================================
  Files          1029     1027       -2     
  Lines         63291    62770     -521     
  Branches       7141     7045      -96     
============================================
+ Hits          35106    35150      +44     
+ Misses        25708    25137     -571     
- Partials       2477     2483       +6     

Impacted Files	Coverage Δ	Complexity Δ
...rg/apache/jmeter/threads/JMeterContextService.java	95.55% <ø> (-0.10%)	16.00 <0.00> (ø)
...n/org/apache/jmeter/threads/JMeterContextHolder.kt	0.00% <0.00%> (ø)	0.00 <0.00> (?)
.../org/apache/jmeter/threads/MultitaskThreadGroup.kt	0.00% <0.00%> (ø)	0.00 <0.00> (?)
.../kotlin/org/apache/jmeter/threads/ThreadContext.kt	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...in/java/org/apache/jmeter/threads/ThreadGroup.java	36.54% <21.73%> (-1.89%)	25.00 <1.00> (+1.00)	⬇️
...n/kotlin/org/apache/jmeter/threads/JMeterThread.kt	62.03% <62.03%> (ø)	54.00 <54.00> (?)
.../org/apache/jmeter/threads/gui/ThreadGroupGui.java	93.00% <100.00%> (+0.29%)	14.00 <0.00> (ø)
...issonarrivals/ConstantPoissonProcessGenerator.java	43.61% <0.00%> (-14.96%)	10.00% <0.00%> (-4.00%)
...g/apache/jmeter/gui/action/LookAndFeelCommand.java	45.28% <0.00%> (-4.17%)	5.00% <0.00%> (-4.00%)
.../src/main/java/org/apache/jmeter/SplashScreen.java	50.00% <0.00%> (-4.06%)	1.00% <0.00%> (-2.00%)
... and 1025 more

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[pmouawad]

Hi @vlsi ,
Do you intend to continue working on this PR which looks really interesting or do you think there are blocking issues ?

Thanks

[vlsi]

@pmouawad , I don't work on this at the moment, however, it probably could fly.