Added a new ResettableInterface and implemented it where possible.

When one use Monolog in a long process like an AMQP worker with a
`FingersCrossedHandler` or `BufferHandler` there is a drawback: as soon as there
is an AMQP message that generate a log >= error (for example), all next AMQP
messages will output logs, even if theses messages don't generate log where
level >= error.

In the same context there is a drawback for processor that add an UUID to the
logs. The UUID should change for each AMQP messages.

---

This patch address this issue with a new interface: `ResettableInterface` interface.
Side note: `reset()`, `flush()`, `clear()`,  are already used in Monolog. So
basically, one can use the `reset()` on the `Logger` and on some
`Handler`s / `Processor`s.

It's especially useful for

* the `FingersCrossedHandler`: it `close()` the buffer, then it `clear()` the buffer.
* the `BufferHandler`: it `flush()` the buffer, then it `clear()` the buffer.
* the `UidProcessor`: it renew the `uid`.

src/Monolog/Handler/BrowserConsoleHandler.php

@@ -69,14 +69,19 @@ class BrowserConsoleHandler extends AbstractProcessingHandler
             } elseif ($format === 'js') {
                 static::writeOutput(static::generateScript());
             }
-            static::reset();
+            static::resetStatic();
         }
     }
 
+    public function reset()
+    {
+        self::resetStatic();
+    }
+
     /**
      * Forget all logged records
      */
-    public static function reset()
+    public static function resetStatic()
     {
         static::$records = array();
     }