[BEAM-115] Update timer/state fields on ParDoPayload to use a map field for consistent name usage

[lukecwik]

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[lukecwik]

R: @tgroh @kennknowles

[tgroh]

LGTM.

[coveralls]

Coverage increased (+0.007%) to 70.576% when pulling e3f83cc on lukecwik:runner_api_proto into 571631a on apache:master.

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