References from a resource-level connection blocks were not returned
from NodeAbstractResource.References, causing the provisioner connection
attributes to sometimes be evaluated too early.
The only reasonable usage of these methods is for them to run concurrently
with other methods, so we mustn't hold a lock to do this work. For tests
that deal with stopping, it's the test's own responsibility to deal with
any concurrency issues that arise from their StopFns running concurrently
with other mock functions.
MockProvider and MockProvisioner implement the new plugin interfaces,
and are built following the patterns used by the legacy
MockResourceProvider and MockResourceProvisioner
