Improve relay draft

This commit brings a compilable, runnable relay draft, with logging
setup, basic README instructions, Mutex protection around shared
resources, implemented with code safety in mind

Notes:
- This is not fully tested
- The mutex design is not the most efficient, and could cause some contention on high traffic
- The REST API is not yet integrated

shell.nix

@@ -0,0 +1,5 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  buildInputs = [ pkgs.websocat ];
+}