Add a publish-subscribe subsystem to publish messages about changes to
OR connections.

connection_or_change_state() in connection_or.c and
control_event_or_conn_event() in control.c publish messages to this
subsystem via helper functions.

Move state constants from connection_or.h to orconn_state.h so that
subscribers don't have to include all of connection_or.h to take
actions based on changes in OR connection state.  Move event constants
from control.h for similar reasons.

Part of ticket 27167.