State for portable timers can interfere with user state

The current active key needs to be set on Flink's state backend to support deleting pending timers on timer registration or firing. A keyed map for pending timers is necessary because Flink only allows deleting/resetting timers by their original time, not by their ids.

Setting the current active key can interfere with accessing user-defined state, but can also interfere when a timer is set at the same time that a timer fires. In cases with a key initialized by the state requests, this would have caused pending timers to be associated with the wrong key which would have led to problems deleting / resetting them.

Related to: BEAM-4681 BEAM-867