decouple raftLog - phase 1: introduce the new type controlflowLog.

[IAMRogerXi]

#142 - this is about to decouple the type raftLog.

since this change may include a lot of cascaded code changes on several types, i'd like to split this refactor into two phase.

phase 1: introduce the type controlflowLog. add it back into raftLog as embedding type.
i move all control flow related fields into type controlflowLog.

phase 2: remove the embedding type. modify type reference in all other files.

in this PR, i implemented phase 1. please review it.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @IAMRogerXi. Thanks for your PR.

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