Use a connection pool for the SimpleHttpClient

In particular I hope this will help the pusher, which makes many requests to
sygnal, and is currently negotiating SSL for each one.

synapse/http/client.py

@@ -30,6 +30,7 @@ from twisted.internet.endpoints import HostnameEndpoint, wrapClientTLS
 from twisted.web.client import (
     BrowserLikeRedirectAgent, ContentDecoderAgent, GzipDecoder, Agent,
     readBody, PartialDownloadError,
+    HTTPConnectionPool,
 )
 from twisted.web.client import FileBodyProducer as TwistedFileBodyProducer
 from twisted.web.http import PotentialDataLoss
@@ -64,13 +65,19 @@ class SimpleHttpClient(object):
     """
     def __init__(self, hs):
         self.hs = hs
+
+        pool = HTTPConnectionPool(reactor)
+        pool.maxPersistentPerHost = 5
+        pool.cachedConnectionTimeout = 2 * 60
+
         # The default context factory in Twisted 14.0.0 (which we require) is
         # BrowserLikePolicyForHTTPS which will do regular cert validation
         # 'like a browser'
         self.agent = Agent(
             reactor,
             connectTimeout=15,
-            contextFactory=hs.get_http_client_context_factory()
+            contextFactory=hs.get_http_client_context_factory(),
+            pool=pool,
         )
         self.user_agent = hs.version_string
         self.clock = hs.get_clock()