We need a little refactoring for this to work, since the
initialization code for the periodic events assumes that libevent is
already initialized, which it can't be until it's configured.
This change, combined with the previous ones, lets other subsystems
declare their own periodic events, without mainloop.c having to know
about them. Implements ticket 30293.
Add a tracker for bootstrap progress, tracking events related to
origin circuit and ORCONN states. This uses the ocirc_event and
orconn_event publish-subscribe subsystems.
Part of ticket 27167.
Add a publish-subscribe subsystem to publish messages about changes to
origin circuits.
Functions in circuitbuild.c and circuitlist.c publish messages to this
subsystem.
Move circuit event constants out of control.h so that subscribers
don't have to include all of control.h to take actions based on
messages they receive.
Part of ticket 27167.
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.
