Shouldn't be process-wide persistent by default

[wizofaus]

Given that Mock repositories are designed for use by unit tests, and MSTest runs unit tests multi-threaded, you get some quite unexpected behavior if you don't pass in "true" for the constructor.
I'd rather see the default behavior be that repository data was persistent per thread, and require a specific option if you wanted it to be process-wide persistent. Either that, or the default behavior should not use a static field-backed datastore.