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`.

tests/Monolog/Handler/FingersCrossedHandlerTest.php

@@ -58,7 +58,7 @@ class FingersCrossedHandlerTest extends TestCase
      * @covers Monolog\Handler\FingersCrossedHandler::activate
      * @covers Monolog\Handler\FingersCrossedHandler::reset
      */
-    public function testHandleRestartBufferingAfterReset()
+    public function testHandleResetBufferingAfterReset()
     {
         $test = new TestHandler();
         $handler = new FingersCrossedHandler($test);
@@ -76,7 +76,7 @@ class FingersCrossedHandlerTest extends TestCase
      * @covers Monolog\Handler\FingersCrossedHandler::handle
      * @covers Monolog\Handler\FingersCrossedHandler::activate
      */
-    public function testHandleRestartBufferingAfterBeingTriggeredWhenStopBufferingIsDisabled()
+    public function testHandleResetBufferingAfterBeingTriggeredWhenStopBufferingIsDisabled()
     {
         $test = new TestHandler();
         $handler = new FingersCrossedHandler($test, Logger::WARNING, 0, false, false);