refactor: split sticky-cookie into a separated feature

TODO.md

@@ -3,6 +3,13 @@
 - Improvement of path matcher
 - More flexible option for rewriting path
 - Refactoring
+
+  Split `backend` module into three parts
+
+  - backend(s): struct containing info, defined for each served domain with multiple paths
+  - upstream/upstream group: information on targeted destinations for each set of (a domain + a path)
+  - load-balance: load balancing mod for a domain + path
+
 - Unit tests
 - Options to serve custom http_error page.
 - Prometheus metrics