[BEAM-1038] Allow stateful DoFn in DataflowRunner

[kennknowles]

Be sure to do all of the following to help us incorporate your contribution
quickly and easily:

• Make sure the PR title is formatted like:
  [BEAM-<Jira issue #>] Description of pull request
• Make sure tests pass via mvn clean verify. (Even better, enable
  Travis-CI on your fork and ensure the whole test matrix passes).
• Replace <Jira issue #> in the title with the actual Jira issue
  number, if there is one.
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---

Confirmed that the post commit should succeed, via the following command:

mvn verify \
    --batch-mode \
    --errors \
    --projects runners/google-cloud-dataflow-java \
    -DforkCount=0 \
    -DfailIfNoTests=false \
    -Dtest=org.apache.beam.sdk.transforms.ParDoTest \
    -DrunnableOnServicePipelineOptions='[
        "--runner=org.apache.beam.runners.dataflow.testing.TestDataflowRunner",
        "--project=...",
        "--tempRoot=..." ]'

with output ending in:

Running org.apache.beam.sdk.transforms.ParDoTest
Tests run: 18, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 273.736 sec - in org.apache.beam.sdk.transforms.ParDoTest

[kennknowles]

R: @bjchambers

[bjchambers]

+1

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Java_MavenInstall/5581/
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