An introduction point is currently rotated when the amount of INTRODUCE2
cells reached a fixed value of 16384. This makes it pretty easy for an
attacker to inflate that number and observe when the IP rotates which leaks
the popularity of the HS (amount of client that passed through the IP).

This commit makes it a random count between the current value of 16384 and
two times that.

Fixes #15745

Signed-off-by: David Goulet <dgoulet@ev0ke.net>

Till someone writes get_interface_address6 interface enumeration that is
routing table aware, these tests will continue to fail on certain
systems because the get_interface_address6() code is broken.

Fixes bug 15591; patch from joelanders

Fixes coverity defect CID 1293337, not in any released version of tor.