BUG-997: fix InMemorySchemaSourceCache interface

It turns out it was impossible to instantiate this class reasonably. Add
a static factory method to alleviate.

Change-Id: Id7ed4976db6ca6303752392c9aded444491edd08
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-997: fix InMemorySchemaSourceCache interface"