OVN: introduce Controller_Event table

OVN: introduce Controller_Event table

Add Controller_Event table to OVN SBDB in order to
report CMS related event.
Introduce event_table hashmap array and controller_event related
structures to ovn-controller in order to track pending events
forwarded by ovs-vswitchd. Moreover integrate event_table hashmap
array with event_table ovn-sbdb table

Signed-off-by: Mark Michelson <mmichels@redhat.com>
Co-authored-by: Mark Michelson <mmichels@redhat.com>
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>