Change Request timeout process

This add a request timeout exception.
It cancels task, when request is timeout.

examples/request_timeout.py

@@ -0,0 +1,21 @@
+from sanic import Sanic
+import asyncio
+from sanic.response import text
+from sanic.config import Config
+from sanic.exceptions import RequestTimeout
+
+Config.REQUEST_TIMEOUT = 1
+app = Sanic(__name__)
+
+
+@app.route("/")
+async def test(request):
+    await asyncio.sleep(3)
+    return text('Hello, world!')
+
+
+@app.exception(RequestTimeout)
+def timeout(request, exception):
+    return text('RequestTimeout from error_handler.')
+
+app.run(host="0.0.0.0", port=8000)