ofproto: Add support for master/slave controller coordination.

ofproto: Add support for master/slave controller coordination.

Now that Open vSwitch has support for multiple simultaneous controllers,
there is some need for a degree of coordination among them.  For now, the
plan is for the controllers themselves to take the lead on this.  This
commit adds a small bit of OVS infrastructure: the ability for a controller
to designate itself as a "master" or a "slave".  There may be at most one
master at a time; when a controller designates itself as the master, then
any existing master is demoted to slave status.  Slave controllers are not
allowed to modify the flow table or global configuration; any attempt to
do so is rejected with a "bad request" error.

Feature #2495.