[BEAM-3882] Fix StateRequestHandler interface to be idiomatic

[axelmagn]

The previous implementation of StateRequestHandler did not conform
to Beam coding conventions. It should not have accepted a return
value as a parameter. This fix updates the interface to instead
return a CompletableStage.

R: @tgroh
CC: @aljoscha, @bsidhom, @lukecwik

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