Increase the ReplayWindow default from 16 to 32.

As a rule, it seems reasonable to make sure that tinc operates correctly
on at least 1G links, since these are pretty common. However, I have
observed replay window issues when operating at speeds of 600 Mbit/s and
above, especially when the receiving end is a Windows system (not sure
why). This commit increases the default so that this won't occur on
fresh setups.