Motivation for this change:
Add DeleteDoc method to the CouchDB layer in order to provide
delete by ID capability.  This function mirrors the delete
capability used by goloveldb.

- Add delete to statecouchdb.go if value is set to null

- Add unit tests for delete in statecouchdb_test.go

Change-Id: I23b8e7e2daf8679dc0fbe8f1a75011530ff8d250
Signed-off-by: Chris Elder <chris.elder@us.ibm.com>