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.

The previous commit prevented evaluating plans against a state snapshot
which is older than the snapshot at which the plan was created.  This is
correct and prevents failures trying to retrieve referenced objects that
may not exist until the plan's snapshot. However, this is insufficient
to guarantee consistency if the following events occur:

1. P1, P2, and P3 are enqueued with snapshot @ 100
2. Leader evaluates and applies Plan P1 with snapshot @ 100
3. Leader evaluates Plan P2 with snapshot+P1 @ 100
4. P1 commits @ 101
4. Leader evaluates applies Plan P3 with snapshot+P2 @ 100

Since only the previous plan is optimistically applied to the state
store, the snapshot used to evaluate a plan may not contain the N-2
plan!

To ensure plans are evaluated and applied serially we must consider all
previous plan's committed indexes when evaluating further plans.

Therefore combined with the last PR, the minimum index at which to
evaluate a plan is:

    min(previousPlanResultIndex, plan.SnapshotIndex)

Rename SnapshotAfter to SnapshotMinIndex. The old name was not
technically accurate. SnapshotAtOrAfter is more accurate, but wordy and
still lacks context about what precisely it is at or after (the index).

SnapshotMinIndex was chosen as it describes the action (snapshot), a
constraint (minimum), and the object of the constraint (index).