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nomad: include snapshot index when submitting plans
Plan application should use a state snapshot at or after the Raft index at which the plan was created otherwise it risks being rejected based on stale data. This commit adds a Plan.SnapshotIndex which is set by workers when submitting plan. SnapshotIndex is set to the Raft index of the snapshot the worker used to generate the plan. Plan.SnapshotIndex plays a similar role to PlanResult.RefreshIndex. While RefreshIndex informs workers their StateStore is behind the leader's, SnapshotIndex is a way to prevent the leader from using a StateStore behind the worker's. Plan.SnapshotIndex should be considered the *lower bound* index for consistently handling plan application. Plans must also be committed serially, so Plan N+1 should use a state snapshot containing Plan N. This is guaranteed for plans *after* the first plan after a leader election. The Raft barrier on leader election ensures the leader's statestore has caught up to the log index at which it was elected. This guarantees its StateStore is at an index > lastPlanIndex.
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