The "Add qualify_timeout processing and eventing" patch introduced
an issue where contacts that had qualify_frequency set to 0 were
showing Unavailable instead Unknown.  This patch checks for
qualify_frequency=0 and create an "Unknown"  contact_status
with an RTT = 0.

Previously, the lack of contact_status implied Unknown but since
we're now changing endpoint state based on contact_status, I've
had to add new UNKNOWN status so that changes could trigger the
appropriate contact_status observers.

ASTERISK-24977: #close

Change-Id: Ifcbc01533ce57f0e4e584b89a395326e098b8fe7