Add a NullCallback that does nothing but delete itself when called
Add a NullCallback that does nothing but delete itself when called

Certain methods require callbacks but sometimes we have none to pass
specially in non-common situations such as mocks/tests.

This adds a callback that can be passed in such situations without
regard to ownership as the callback deletes itself when called.

Change-Id: Id5d4a30c724804b1338d7228f4d34d1c3b5a212d
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4160
Tested-by: jenkins
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>

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