dos: Make circuit rate limit per second, not tenths anymore

Because this touches too many commits at once, it is made into one single
commit.

Remove the use of "tenths" for the circuit rate to simplify things. We can
only refill the buckets at best once every second because of the use of
approx_time() and our token system is set to be 1 token = 1 circuit so make
the rate a flat integer of circuit per second.

Signed-off-by: David Goulet <dgoulet@torproject.org>