Give each or_connection_t a slightly randomized idle_timeout

Instead of killing an or_connection_t that has had no circuits for
the last 3 minutes, give every or_connection_t a randomized timeout,
so that an observer can't so easily infer from the connection close
time the time at which its last circuit closed.

Also, increase the base timeout for canonical connections from 3
minutes to 15 minutes.

Fix for ticket 6799.
Replace last_added_nonpadding with last_had_circuits

The point of the "idle timeout" for connections is to kill the
connection a while after it has no more circuits.  But using "last
added a non-padding cell" as a proxy for that is wrong, since if the
last circuit is closed from the other side of the connection, we
will not have sent anything on that connection since well before the
last circuit closed.

This is part of fixing 6799.

When applied to 0.2.5, it is also a fix for 12023.
Tweak changes entry for 6799
Merge remote-tracking branch 'public/ticket6799_024_v2_squashed'

Conflicts:
	src/or/channel.c
	src/or/circuitlist.c
	src/or/connection.c

Conflicts involved removal of next_circ_id and addition of
unusable-circid tracking.
whitespaces fixes