Co-Authored-By: Mahmood Ali <mahmood@notnoop.com>

Co-Authored-By: Michael Schurter <michael.schurter@gmail.com>

devcluster: Add standalone server

client: drop unused DC field from servers list

client: synchronize client.invalidAllocs access

invalidAllocs may be accessed and manipulated from different goroutines,
so must be locked.

Document `nomad exec` command

sort and changelog entries

In each section, entries should be sorted by component name (except for
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This adds a single-node configuration of a dev cluster, for use when
testing things that require state restoration or multiple clients, but don't
necessarily require a 3+ node cluster.

client: Pass servers contacted ch to allocrunner

This fixes an issue where batch and service workloads would never be
restarted due to indefinitely blocking on a nil channel.

It also raises the restoration logging message to `Info` to simplify log
analysis.