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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