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.

sample-lnd.conf

@@ -1142,6 +1142,11 @@ litecoin.node=ltcd
 ; probability (default: 1h0m0s)
 ; routerrpc.apriori.penaltyhalflife=2h
 
+; Defines the fraction of channels' capacities that is considered liquid in
+; pathfinding, a value between [0.75-1.0]. A value of 1.0 disables this
+; feature. (default: 0.75)
+; routerrpc.apriori.capacityfraction=0.9
+
 ; Describes the scale over which channels still have some liquidity left on
 ; both channel ends. A very low value (compared to typical channel capacities)
 ; means that we assume unbalanced channels, a very high value means randomly