Flink user state GC depends on order of timer firing

The newly added ParDoTest.testEventTimeTimerMultipleKeys() fails because the DoFn sets a timer for window.maxTimestamp() which also happens to be the GC timer for the user state. The Flink Runner uses timers to schedule GC, the user-set timer and the GC timer have a different timer id, so they don't clash. However, if the GC timer is being processed before the user timer then the user doesn't have a chance to access the state anymore because it will already be cleared out by the time the user timer is being processed.