[BEAM-1641] Use processing time for synchronized processing time in Flink runner

[kennknowles]

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There are some esoteric expectations that this might violate, like exactly one hourly downstream output aligned to an hour, versus a cfewuple, but we should rephrase docs to remove those expectations IMO.

R: @aljoscha @JingsongLi

[aljoscha]

Run Flink ValidatesRunner

[aljoscha]

LGTM! once Jenkins is green

Thanks for fixing this, @kennknowles! 👍

[kennknowles]

Worker beam6 has been unhappy for a while. Since this affect only Flink and the ValidatesRunner passed, I will also run the thorough verify manually I guess.

[kennknowles]

retest this please

[coveralls]

Coverage increased (+0.003%) to 69.856% when pulling 05722ac on kennknowles:Flink-sync-proc into ae72456 on apache:master.