fix: add backpressure to GroupBy [WriteApi]

[bednar]

Closes #168
Closes #202
Closes #357

Proposed Changes

This PR fixes missing backpressure in groupBy operator after update RxJava to `v3.

For more info see:

• https://github.com/ReactiveX/RxJava/wiki/What's-different-in-3.0#groupby-backpressure-example
• Asynchronous write fails when writing data to multiple buckets #202 (comment)

Testing Snapshot

Testing snapshots are deployed into OSS Snapshot repository as a version 6.2.0.groupBy-SNAPSHOT.

Maven

<repository>
    <id>ossrh</id>
    <name>OSS Snapshot repository</name>
    <url>https://oss.sonatype.org/content/repositories/snapshots/</url>
    <releases>
        <enabled>false</enabled>
    </releases>
    <snapshots>
        <enabled>true</enabled>
    </snapshots>
</repository>

Gradle

repositories {
    maven { url "https://oss.sonatype.org/content/repositories/snapshots" }
}

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• mvn test completes successfully
• Commit messages are conventional
• Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

Merging #358 (76ccc75) into master (77fb8db) will decrease coverage by 0.72%.
The diff coverage is 78.08%.

@@             Coverage Diff              @@
##             master     #358      +/-   ##
============================================
- Coverage     89.63%   88.91%   -0.73%     
- Complexity      729      778      +49     
============================================
  Files           167      169       +2     
  Lines          6590     6864     +274     
  Branches        327      364      +37     
============================================
+ Hits           5907     6103     +196     
- Misses          591      653      +62     
- Partials         92      108      +16     

Impacted Files	Coverage Δ
...nternal/flowable/FlowableBufferTimedFlushable.java	70.68% <70.68%> (ø)
...al/flowable/BackpressureBatchesBufferStrategy.java	75.57% <75.57%> (ø)
.../influxdb/client/internal/AbstractWriteClient.java	89.47% <92.42%> (-5.27%)	⬇️
...nfluxdb/client/write/events/BackpressureEvent.java	100.00% <100.00%> (ø)

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