Add monitor object creation.

Now keeps track of all Monitor objects created via a 'Subscribe' message
from a client on a given socket connection. There may be multiple
subscribe requests which come in on a connection from a monitor client
application.

New objects added with this commit:

* Connection_t: One per client connection. All Monitor subscriptions for
  a client connection are help in this object. When the connection is closed,
  all monitor subscriptions over that connection are removed (unsubscribed)/

* Monitor_t: Tracks a single subscription. Removed when a 'Unsubscribe' message
  is received from the monitor client application or when the connection to
  the client is closed.