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/Processor/UidProcessor.php

@@ -11,12 +11,14 @@
 
 namespace Monolog\Processor;
 
+use Monolog\ResettableInterface;
+
 /**
  * Adds a unique identifier into records
  *
  * @author Simon Mönch <sm@webfactory.de>
  */
-class UidProcessor
+class UidProcessor implements ResettableInterface
 {
     private $uid;
 
@@ -26,7 +28,8 @@ class UidProcessor
             throw new \InvalidArgumentException('The uid length must be an integer between 1 and 32');
         }
 
-        $this->uid = substr(hash('md5', uniqid('', true)), 0, $length);
+
+        $this->uid = $this->generateUid($length);
     }
 
     public function __invoke(array $record)
@@ -43,4 +46,14 @@ class UidProcessor
     {
         return $this->uid;
     }
+
+    public function reset()
+    {
+        $this->uid = $this->generateUid(strlen($this->uid));
+    }
+
+    private function generateUid($length)
+    {
+        return substr(hash('md5', uniqid('', true)), 0, $length);
+    }
 }