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/GroupHandler.php

@@ -12,6 +12,7 @@
 namespace Monolog\Handler;
 
 use Monolog\Formatter\FormatterInterface;
+use Monolog\ResettableInterface;
 
 /**
  * Forwards records to multiple handlers
@@ -90,6 +91,17 @@ class GroupHandler extends AbstractHandler
         }
     }
 
+    public function reset()
+    {
+        parent::reset();
+
+        foreach ($this->handlers as $handler) {
+            if ($handler instanceof ResettableInterface) {
+                $handler->reset();
+            }
+        }
+    }
+
     /**
      * {@inheritdoc}
      */