Optionally use uvloop for clients

uvloop is a bit faster than the standard asyncion event loop.

Unfortunately, there's an issue with the ZEO tests and using uvloop in
the ZEO server:

  MagicStack/uvloop#39

Fortunately, most of the performamce benefits of using uvloop seems to
be in the client.

For now, we'll just use ivloop on the client side, as the incremental
effort of using it in the server aren't worth futher wrestling with
the tests. (I spent quite a bit of time just narrowing down the cause
of the test issue.)

With this PR, if uvloop can be imported, it will be used for the
client. (uvloop requires Python 3.5 or later and doesn't support Windows.)

.travis.yml

@@ -18,14 +18,14 @@ matrix:
           python: 2.7
           env: ZEO4_SERVER=1
         - os: linux
-          python: 3.4
+          python: 3.5
           env: ZEO4_SERVER=1
         - os: linux
           python: 3.5
-          env: ZEO4_SERVER=1
+          env: BUILOUT_OPTIONS=extra=,uvloop
 install:
     - pip install zc.buildout
-    - buildout
+    - buildout $BUILOUT_OPTIONS
 cache:
   directories:
     - eggs

src/ZEO/asyncio/client.py

@@ -2,8 +2,13 @@
 
 if PY3:
     import asyncio
+    try:
+        from uvloop import new_event_loop
+    except ImportError:
+        from asyncio import new_event_loop
 else:
     import trollius as asyncio
+    from trollius import new_event_loop
 
 from ZEO.Exceptions import ClientDisconnected, ServerException
 import concurrent.futures
@@ -836,7 +841,7 @@ def __init__(self, addrs, client, cache,
     def run(self):
         loop = None
         try:
-            loop = asyncio.new_event_loop()
+            loop = new_event_loop()
             self.setup_delegation(loop)
             self.started.set()
             loop.run_forever()

src/ZEO/tests/testssl.py

@@ -120,6 +120,7 @@ def test_ssl_pw(self):
 @mock.patch(('asyncio' if PY3 else 'trollius') + '.async')
 @mock.patch(('asyncio' if PY3 else 'trollius') + '.set_event_loop')
 @mock.patch(('asyncio' if PY3 else 'trollius') + '.new_event_loop')
+@mock.patch('ZEO.asyncio.client.new_event_loop')
 class SSLConfigTestMockiavellian(ZEOConfigTestBase):
 
     @mock.patch('ssl.create_default_context')