Avoid duplicate URL parsing for async connections (#958)

With this commit we provide a URL object when issuing a request instead
of a string representation to avoid expensive string parsing in aiohttp.
In our tests this has reduced the client side overhead by about one
millisecond which is important when benchmarking queries which finish
within single-digit milliseconds.

Relates #944

esrally/async_connection.py

@@ -5,6 +5,7 @@
 import aiohttp
 from aiohttp.client_exceptions import ServerFingerprintMismatch
 import async_timeout
+import yarl
 
 from elasticsearch.exceptions import ConnectionError, ConnectionTimeout, ImproperlyConfigured, SSLError
 from elasticsearch.connection import Connection
@@ -97,11 +98,7 @@ def __init__(self, host='localhost', port=9200, http_auth=None,
             trace_configs=trace_configs,
             response_class=RawClientResponse
         )
-
-        self.base_url = 'http%s://%s:%d%s' % (
-            's' if use_ssl else '',
-            host, port, self.url_prefix
-        )
+        self.scheme = "https" if use_ssl else "http"
 
     @asyncio.coroutine
     def close(self):
@@ -111,8 +108,16 @@ def close(self):
     def perform_request(self, method, url, params=None, body=None, timeout=None, ignore=(), headers=None):
         url_path = url
         if params:
-            url_path = '%s?%s' % (url, urlencode(params or {}))
-        url = self.base_url + url_path
+            query_string = urlencode(params)
+        else:
+            query_string = ""
+        # Provide correct URL object to avoid string parsing in low-level code
+        url = yarl.URL.build(scheme=self.scheme,
+                             host=self.hostname,
+                             port=self.port,
+                             path=url,
+                             query_string=query_string,
+                             encoded=True)
 
         start = self.loop.time()
         response = None