Changes SplittableDoFn to call TruncateRestriction on drain

[acrites]

Previous behavior was to call processElement on the currentRestriction and then not set any continuation timers (which causes the pipeline to eventually complete draining). With this change, we instead first call the DoFn's TruncateRestriction method. The default behavior for unbounded restrictions is to return null, which should finish immediately without processing. If the method returns a non-null residual restriction, we pass that to ProcessElement.

Also added some unit tests verifying that the default TruncateRestriction is used, or that the overridden one gets called.

This mimics the behavior of the FnApi harness found here:

beam/sdks/java/harness/src/main/java/org/apache/beam/fn/harness/SplittableTruncateSizedRestrictionsDoFnRunner.java

Line 349 in 2ee432b

try {

.

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