Added asynchronous connection for stats socket

The stats socket now allows an option to connect asynchronously in
the same manner as the api socket  connection. New method
AsyncConnectStats returns a channel where notificaitons
of type ConnectionEvent will be sent.

Fixed the stats reconnect procedure which sometimes failed to re-eneable
the connection.

Change-Id: I0bdb19f0d57e3a1ea259b8b1ba0a5e5fa49a09db
Signed-off-by: Vladimir Lavor <vlavor@cisco.com>