Rework AsyncDoFn/FileDownloadDoFn to avoid outputting element outside of allowed timestamp bound

[clairemcginty]

See discussion here: #5806
And here: apache/beam#34902 (comment)
And here: apache/beam#36838 (comment)

On updating to Beam 2.69, AsyncDoFn/FileDownloadDoFn tests started failing with:

[info] - should propagate element metadata *** FAILED *** (45 milliseconds)
[info]   org.apache.beam.sdk.Pipeline$PipelineExecutionException: java.lang.IllegalArgumentException: Cannot output with timestamp 1970-01-01T00:00:00.001Z. Output timestamps must be no earlier than the timestamp of the current input or timer (1970-01-01T00:00:00.008Z) minus the allowed skew (0 milliseconds) and no later than 294247-01-10T04:00:54.775Z. See the DoFn#getAllowedTimestampSkew() Javadoc for details on changing the allowed skew.
[info]   at org.apache.beam.runners.direct.DirectRunner$DirectPipelineResult.waitUntilFinish(DirectRunner.java:377)
[info]   at org.apache.beam.runners.direct.DirectRunner$DirectPipelineResult.waitUntilFinish(DirectRunner.java:345)
[info]   at org.apache.beam.runners.direct.DirectRunner.run(DirectRunner.java:218)
[info]   at org.apache.beam.runners.direct.DirectRunner.run(DirectRunner.java:67)
[info]   at org.apache.beam.sdk.Pipeline.run(Pipeline.java:325)
[info]   at org.apache.beam.sdk.testing.TestPipeline.run(TestPipeline.java:442)
[info]   at org.apache.beam.sdk.testing.TestPipeline.run(TestPipeline.java:381)
[info]   at com.spotify.scio.ScioContext.execute(ScioContext.scala:671)
[info]   at com.spotify.scio.ScioContext$$anonfun$run$1.apply(ScioContext.scala:658)
[info]   at com.spotify.scio.ScioContext$$anonfun$run$1.apply(ScioContext.scala:646)
[info]   ...
[info]   Cause: java.lang.IllegalArgumentException: Cannot output with timestamp 1970-01-01T00:00:00.001Z. Output timestamps must be no earlier than the timestamp of the current input or timer (1970-01-01T00:00:00.008Z) minus the allowed skew (0 milliseconds) and no later than 294247-01-10T04:00:54.775Z. See the DoFn#getAllowedTimestampSkew() Javadoc for details on changing the allowed skew.
[info]   at org.apache.beam.runners.core.SimpleDoFnRunner.checkTimestamp(SimpleDoFnRunner.java:263)
[info]   at org.apache.beam.runners.core.SimpleDoFnRunner.access$1300(SimpleDoFnRunner.java:89)
[info]   at org.apache.beam.runners.core.SimpleDoFnRunner$DoFnProcessContext.lambda$outputWindowedValue$0(SimpleDoFnRunner.java:462)
[info]   at org.apache.beam.sdk.values.WindowedValues$Builder.output(WindowedValues.java:210)
[info]   at org.apache.beam.runners.core.SimpleDoFnRunner$DoFnProcessContext.outputWindowedValue(SimpleDoFnRunner.java:465)
[info]   at org.apache.beam.sdk.transforms.DoFnOutputReceivers$WindowedContextOutputReceiver.output(DoFnOutputReceivers.java:123)
[info]   at org.apache.beam.sdk.values.WindowedValues$Builder.output(WindowedValues.java:210)
[info]   at org.apache.beam.sdk.transforms.DoFn$OutputReceiver.outputWindowedValue(DoFn.java:416)
[info]   at com.spotify.scio.transforms.BaseAsyncLookupDoFn.lambda$processElement$0(BaseAsyncLookupDoFn.java:190)
[info]   at com.spotify.scio.transforms.BaseAsyncLookupDoFn.flush(BaseAsyncLookupDoFn.java:310)
[info]   ...

Per discussion in the linked PRs/issues, we worked around this by just overriding allowed timestamp skew in those DoFns. However, a better option may be to just output the completed elements entirely in FinishBundle rather than in both ProcessElement/FinishBundle, which should (™️) avoid the timestamp check added to ProcessElement.