ofproto: Use ofproto_mutex for groups and keep track of referring flows.

ofproto: Use ofproto_mutex for groups and keep track of referring flows.

Adding groups support for bundles is simpler if also groups are
modified under ofproto_mutex.

Eliminate the search for rules when deleting a group so that we will
not keep the mutex for too long.

Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>