Reduce default max_channel_saturation_power_of_half to 2 (max 1/4)

Reduce default max_channel_saturation_power_of_half to 2 (max 1/4)

Saturating a channel beyond 1/4 of its capacity seems like a more
reasonable threshold for avoiding a path than 1/2, especially given
we should still be willing to send a payment with a lower
saturation limit if it comes to that.

This requires an (obvious) change to some router tests, but also
requires a change to the `fake_network_test`, opting to simply
remove some over-limit test code there - `fake_network_test` was
our first ever functional test, and while it worked great to ensure
LDK worked at all on day one, we now have a rather large breadth
of functional tests, and a broad "does it work at all" test is no
longer all that useful.