[BEAM-7901] Increase default chunk size for gRPC commit and get data streams.

[slavachernyak]

The initial choice of chunk size was arbitrary, and there is evidence from testing that larger chunks improve performance.

R: @lukecwik

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---		---	---
Java
Python		---			---	---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

[lukecwik]

Will merge when tests are green.

[lukecwik]

Looks like there is a failing test

org.apache.beam.runners.dataflow.worker.windmill.GrpcWindmillServerTest.testStreamingGetDataHeartbeats
Error Message
java.lang.AssertionError: 
Expected: a value less than or equal to <64512>
     but: <108273> was greater than <64512>

https://builds.apache.org/job/beam_PreCommit_Java_Commit/7072/testReport/junit/org.apache.beam.runners.dataflow.worker.windmill/GrpcWindmillServerTest/testStreamingGetDataHeartbeats/