This commit add the hs_dos.{c|h} file that has the purpose of having the
anti-DoS code for onion services.

At this commit, it only has one which is a function that decides if an
INTRODUCE2 can be sent on the given introduction service circuit (S<->IP)
using a simple token bucket.

The rate per second is 25 and allowed burst to 200.

Basic defenses on #15516.

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