This extracts existing code from Client and adds tests. The glue code is
far from ideal and highlights the amount of work needed to decouple Client
from so many parts of the system.
* src/client/Client.js (_dapiManager): New field.
(_init): Use DataApiMediator.
(_createProgram): Assign `_dapiManager' (this is not at all
ideal). Remove hooks from it: fieldLoading, updateFieldData,
clearFieldData.
* src/client/ClientDependencyFactory.js (createDataApiMediator): New alias
to DataApiMediator constructor.
* src/client/dapi/DataApiMediator.js: New class.
* test/client/dapi/DataApiMediatorTest.js: New test case.
DEV-3257
Mike Gerwitz