routing: implement bimodal probability estimator

Implements a new probability estimator based on a probability theory
framework.

The computed probability consists of:
* the direct channel probability, which is estimated based on a
  depleted liquidity distribution model, formulas and broader concept derived
  after Pickhardt et al. https://arxiv.org/abs/2103.08576
* an extension of the probability model to incorporate knowledge decay
  after time for previous successes and failures
* a mixed node probability taking into account successes/failures on other
  channels of the node (similar to the apriori approach)

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

@@ -389,6 +389,16 @@ in the lnwire package](https://github.com/lightningnetwork/lnd/pull/7303)
 
 * [Pathfinding takes capacity of edges into account to improve success
   probability estimation.](https://github.com/lightningnetwork/lnd/pull/6857)
+* [A new probability model ("bimodal") is added which models channel based
+  liquidities within a probability theory framework.](
+  https://github.com/lightningnetwork/lnd/pull/6815)
+
+## Configuration
+* Note that [this pathfinding change](https://github.com/lightningnetwork/lnd/pull/6815)
+  introduces a breaking change in lnd.conf apriori parameters under the routing
+  section, see sample-lnd.conf for an updated configuration. The behavior of
+  `lncli setmccfg/getmccfg` is altered as well.
+
 
 ### Tooling and documentation
 
@@ -445,6 +455,7 @@ refactor the itest for code health and maintenance.
 * Alyssa Hertig
 * andreihod
 * Antoni Spaanderman
+* bitromortac
 * Carla Kirk-Cohen
 * Carsten Otto
 * Chris Geihsler