Remove cyclic references in HTTP implementations (#1604)

* Remove parser reference to break the cyclic reference to protocol and let garbage colletor to do it's work

* Unset keepalive after closing transport

uvicorn/protocols/http/h11_impl.py

@@ -145,6 +145,7 @@ def connection_lost(self, exc: Optional[Exception]) -> None:
             self.flow.resume_writing()
         if exc is None:
             self.transport.close()
+            self._unset_keepalive_if_required()
 
     def eof_received(self) -> None:
         pass

uvicorn/protocols/http/httptools_impl.py

@@ -137,6 +137,9 @@ def connection_lost(self, exc: Optional[Exception]) -> None:
             self.flow.resume_writing()
         if exc is None:
             self.transport.close()
+            self._unset_keepalive_if_required()
+
+        self.parser = None
 
     def eof_received(self) -> None:
         pass