Backport of fix deadlock in plan_apply into release/1.2.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #13407 to be assessed for backporting due to the inclusion of the label backport/1.2.x.

The below text is copied from the body of the original PR.

---

The plan applier has to get a snapshot with a minimum index for the
plan it's working on in order to ensure consistency. Under heavy raft
loads, we can exceed the timeout. When this happens, we hit a bug
where the plan applier blocks waiting on the indexCh forever, and
all schedulers will block in Plan.Submit.

Closing the indexCh when the asyncPlanWait is done with it will
prevent the deadlock without impacting correctness of the previous
snapshot index.

---

Note for reviewers: the test code here can't actually exercise this bug,
because there's no interface for us to introduce this timeout. But this
code path was not covered by unit testing, so with these tests we at least
exercise the normal state.

6fab937 is how we verified the behavior and that the fix works, but we'll
want to circle back later to build out some fault injection into the state
store code at some point.

The deadlocked goroutine stack looks like this:

goroutine 625541538 [chan receive, 514 minutes]:
github.com/hashicorp/nomad/nomad.(*planner).planApply(0xc000a410a0)
	github.com/hashicorp/nomad/nomad/plan_apply.go:156 +0x35e
created by github.com/hashicorp/nomad/nomad.(*Server).establishLeadership
	github.com/hashicorp/nomad/nomad/leader.go:250 +0x21e

cc @jazzyfresh @lgfa29 @mikenomitch
Fixes #10289

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