raft: fix TLA+ specification to match unapplied config guard

[CharlieQiu2017]

As described in issue #456, the current TLA+ model of this Raft implementation is subtly broken due to a missing guard condition for leader campaigns. In the actual implementation, a follower or pre-candidate needs to check that there are no unapplied committed reconfig entries in its local log before initiating a campaign, but this check is not enforced in the model, leading to a potential safety violation. This PR fixes the issue by adding the guard condition. Running the driver spec in #456 against the updated TLA+ model shows that the safety violation is avoided.

[kubernetes-prow]

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This pull-request has been approved by: CharlieQiu2017
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[kubernetes-prow]

Hi @CharlieQiu2017. Thanks for your PR.

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[ahrtr]

cc @joshuazh-x