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 Support for Receiving Private CLASS/RRDATA Messages

*Agent parses message with OPCODE 14 and separate CLASS, RRDATA
fields and calls for a create and delete event, respectively.
*There isn't much done at the moment, but they will call into
the pluggable backend when implemented.

Change-Id: I48c2deb729c853bebec74a04b6c97c89426000c1
Agent - Add Backend Capabilities

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

Change-Id: I730e666bbd9db48d1f6793f8f6b48decaee39cb9