Add PortMapper to map port-names to port-numbers

PortMapper gets current list of ports when connection
is established and keeps it updated based on port-state
notifications.

Other changes:
* Minor change to message-handler interface
* Fix a race in SendMessage. ofConn could get
destroyed between the IsConnected() check and use
ofConn in vconn_send(), leading to bad memory access. Fix is to
check and use in the same lock scope.

Change-Id: I2aa5cb907cb800931dacba20a951f6a504749855
Signed-off-by: Amit Bose <bose@noironetworks.com>