Track "uncleanly unregistered" hosts on the master HC; expose via management API

When the master HC detects that a slave HC is no longer available but hasn't properly unregistered, the slave should still be visible in the management API but with some sort of status indicator.

The slave should probably not remaining in the regular management tree, since that will encourage clients to invoke operations that will just fail, but it's existence and "unknown" status should be reflected in some manner accessible to management clients who wish to obtain a more complete view of the domain topology.