Use a Var instead of an Atom for system state

The Atom's swap! function may run its provided function multiple
times. Because of this, using an Atom for the system state can
occasionally result in duplicate TCP connections being opened when the
system is started. These connections remain open until they
timeout (this is typically 5 minutes for IRC).

This commit changes the container for the system state from an Atom to a
Var, whose alter-var-root function (analogous to swap!) does not have
this issue.

Signed-off-by: Sebastian Crane <seabass-labrax@gmx.com>

README.md

@@ -66,18 +66,18 @@ Since `matchbot` uses Clojure's [tools.deps library](https://clojure.org/guides/
 You can create, start and stop an instance of the chatbot process with the functions in the `system` namespace:
 
 ``` clojure
-;; creating a new instance
-(def my-instance (atom (system/system)))
+;; creating a new instance - an empty Var
+(def my-instance nil)
 
 ;; starting the instance
-(swap! my-instance system/start)
+(alter-var-root #'my-instance system/start)
 
 ;; restarting the instance
-(system/restart my-instance)
+(system/restart #'my-instance)
 
 ;; stopping and resetting the instance
-(swap! my-instance stop)
-(reset! my-instance (system/system))
+(alter-var-root #'my-instance system/stop)
+(alter-var-root #'my-instance (constantly nil))
 ```
 
 Once you are familiar with nREPL, you can additionally use [tools.namespace.repl](https://github.com/clojure/tools.namespace) to make reevaluating (reloading) your changes easier:

src/system.clj

@@ -55,11 +55,11 @@
      (deref (:state system)))
     (irclj.core/quit (system :irc))))
 
-(defn restart [system-atom]
+(defn restart [system-var]
   (do
-    (swap! system-atom stop)
-    (reset! system-atom (system))
-    (swap! system-atom start)))
+    (alter-var-root system-var stop)
+    (alter-var-root system-var (constantly system))
+    (alter-var-root system-var start)))
 
 (defn -main [& args]
   (let [main-system (system/start nil)]