vswitchd: Add "enable-async-messages" setting for controllers.

vswitchd: Add "enable-async-messages" setting for controllers.

An OpenFlow connection can start receiving asynchronous messages such as
"packet-ins" immediately at connect time.  If there is a lot of traffic on
the network then this can swamp the controller before it gets a chance to
set up an initial flow table.  This setting overrides this OpenFlow
behavior, changing it so that the connection initially enables no
asynchronous messages at all.  The controller can enable any messages that
it actually wants when it is ready for them.

Feature #7086.
Signed-off-by: Ben Pfaff <blp@nicira.com>