SCHEMA-133. Make KijiDataRequest immutable and add a Builder class

* Adds KijiDataRequestBuilder. Get an instance by calling KijiDataRequest.builder().
* KijiDataRequests are immutable; all fields of requests and their columns are final.
* KijiDataRequests are created via KijiDataRequest.create() or KijiDataRequestBuilder.
* Semantics of KijiDataRequest.merge() and KijiDataRequestBuilder are specified in their javadoc.
* Added new unit tests
* Updated all uses of KijiDataRequest API

Signed-off-by: Christophe Taton <taton@wibidata.com>