[BEAM-3052] Set end-of-window timer in AfterWatermarkStateMachine

[kennknowles]

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The state machines for the variants of AfterWatermark.pastEndOfWindow() were inconsistent in when they set a timer or not. Since this state machine should be decoupled from whatever context it is being run in, it should set a timer for the time it is interested in.

This is deliberately peeled off #3988 since it is simpler and more definitely correct. The other bits in that PR are under further revision.

R: @bjchambers

[bjchambers]

Are there additional tests you can add to prevent future regressions to these?

[kennknowles]

Now that you mention it, yes. The TriggerTester sets this timer unconditionally because the ReduceFnRunner manages it. So removing that should cause some failing test without this change.

[kennknowles]

Added a failing test, checked that it failed without the current change.

[coveralls]

Changes Unknown when pulling 3d36f63 on kennknowles:AfterWatermark-timer into ** on apache:master**.