[BEAM-6732] JdbcIO: added "Write.withResults()"

[aromanenko-dev]

• Added new transform called WriteVoid which supplies PCollection<Void> as output;
• Moved all implementation of Write transform into WriteVoid;
• Made all methods of Write to be the wrappers around WriteVoid as internal writing transform, no backwards incompatible changes for public API;
• Added withResults() to Write which returns WriteVoid transform;
• Some minor tests refactoring.

R: @iemejia @jbonofre

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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	---		---	---

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[jbonofre]

LGTM thanks !