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:

  https://github.com/MagicStack/uvloop/issues/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.)