routing+lnrpc: make capacity factor configurable

We make the capacity factor configurable via an lnd.conf routerrpc
apriori parameter. The capacity factor trades off increased success
probability with a reduced set of channel candidates, which may lead to
increased fees. To let users choose whether the factor is active or not,
we add a config setting where a capacity fraction of 1.0 disables the
factor. We limit the capacity fraction to values between 0.75 and 1.0.
Lower values may discard too many channels.

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

@@ -433,6 +433,8 @@ in the lnwire package](https://github.com/lightningnetwork/lnd/pull/7303)
 * [A new probability model ("bimodal") is added which models channel based
   liquidities within a probability theory framework.](
   https://github.com/lightningnetwork/lnd/pull/6815)
+* [The a priori capacity factor is made configurable and its effect is 
+  limited.](https://github.com/lightningnetwork/lnd/pull/7444)
 
 ## Configuration
 * Note that [this pathfinding change](https://github.com/lightningnetwork/lnd/pull/6815)