Made Promise have a virtual destructor.

See summary.

From: Benjamin Hindman <benh@berkeley.edu>
Review: https://reviews.apache.org/r/14382
Added the registrar.

From: Benjamin Hindman <benh@berkeley.edu>
Review: https://reviews.apache.org/r/14383
Injected Registrar into the Master and the tests.

From: Benjamin Hindman <benh@berkeley.edu>
Review: https://reviews.apache.org/r/14382