routing: apply capacity factor

We multiply the apriori probability with a factor to take capacity into
account:

P *= 1 - 1 / [1 + exp(-(amount - cutoff)/smearing)]

The factor is a function value between 1 (small amount) and 0 (high
amount). The zero limit may not be reached exactly depending on the
smearing and cutoff combination. The function is a logistic function
mirrored about the y-axis. The cutoff determines the amount at which a
significant reduction in probability takes place and the smearing
parameter defines how smooth the transition from 1 to 0 is. Both, the
cutoff and smearing parameters are defined in terms of fixed fractions
of the capacity.

docs/release-notes/release-notes-0.16.0.md

@@ -260,6 +260,11 @@ certain large transactions](https://github.com/lightningnetwork/lnd/pull/7100).
   using the address in question](
   https://github.com/lightningnetwork/lnd/pull/7025)
 
+## Pathfinding
+
+* [Pathfinding takes capacity of edges into account to better estimate the
+  success probability.](https://github.com/lightningnetwork/lnd/pull/6857)
+
 ### Tooling and documentation
 
 * [The `golangci-lint` tool was updated to