When a system or sysbatch job specify constraints that none of the
current nodes meet, report a warning to the user.

Also, for sysbatch job, mark the job as dead as a result.

A sample run would look like:

```
$ nomad job run ./example.nomad
==> 2021-08-31T16:57:35-04:00: Monitoring evaluation "b48e8882"
    2021-08-31T16:57:35-04:00: Evaluation triggered by job "example"
==> 2021-08-31T16:57:36-04:00: Monitoring evaluation "b48e8882"
    2021-08-31T16:57:36-04:00: Evaluation status changed: "pending" -> "complete"
==> 2021-08-31T16:57:36-04:00: Evaluation "b48e8882" finished with status "complete" but failed to place all allocations:
    2021-08-31T16:57:36-04:00: Task Group "cache" (failed to place 1 allocation):
      * Constraint "${meta.tag} = bar": 2 nodes excluded by filter
      * Constraint "${attr.kernel.name} = linux": 1 nodes excluded by filter

$ nomad job status example
ID            = example
Name          = example
Submit Date   = 2021-08-31T16:57:35-04:00
Type          = sysbatch
Priority      = 50
Datacenters   = dc1
Namespace     = default
Status        = dead
Periodic      = false
Parameterized = false

Summary
Task Group  Queued  Starting  Running  Failed  Complete  Lost
cache       0       0         0        0       0         0

Allocations
No allocations placed
```

… tests

Defensively deep copy AllocMetric to avoid side effects from shared map
references.

While I don't think this fully encompasses the changes, other bits
like marking sysbatch as dead immediately are new so haven't changed
from a previous release.