Agent - Add basic NOTIFY support

*Can get NOTIFYs but do nothing with them

Change-Id: I5905f27ff704783f0c67e5483654bbec7f6831eb
Agent - Add AXFR Capability

*When a NOTIFY comes in and verifed an AXFR is triggered
*Adds a concept of Masters, separate from "allow_notify"

Change-Id: Ia7d70561cdae73c7cca956f821fc3b11736b087c
Agent - Add Backend Capabilities

*Create a Base Backend
*Call into it on NOTIFY, CREATE, DELETE events
*Port a Bind9 Backend

Change-Id: I730e666bbd9db48d1f6793f8f6b48decaee39cb9