Avoid waiting on futures using timeouts -> major performance win on Python 2

When data are loaded, we notify waiting code before updating the cache.

This can confuse white-box tests that make assertions about the client
cache after a load, because the cache change may or may not have been
made yet.

By making an unimportant assertion less restrictive.

On Python 2, waiting on a future result with a timeout is very
expensive. (Because conditions with timeouts are very expensive.)

The thing is, we still want to use timeouts when we're disconnected.
We don't want requests to fail right away of the disconnection is
short.

We do a bit of a dance to only use a timeout if we're disconnected.

The ad-hoc server no longer leaves a configuration file behind.

Exit with failure codes instead of success codes on unsupported Python versions.

Fixes #75.

Zeo 5

Conflicts:
	.travis.yml
	CHANGES.rst
	setup.py
	src/ZEO/StorageServer.py
	src/ZEO/cache.py

Asyncio-based ZEO client and server (2)

Got uvloop working with the mtacceptor

thanks to the new connect_accepted_socket API.

Abstracted the code that supports using asyncio vs trollius vs uvloop

Use uvloop in the single-threaded server

When creating an ad hoc server, a log file isn't created by default

To accomplish this, it was necessary to rearrange the tests so that
tests that ran servers in threads rather than using multiprocessing
into their own layer.  This is due to a bug currently in uvloop that
prevents using running uvloop servers in a process and in subprocesses
created with multiprocessing:

https://github.com/MagicStack/uvloop/issues/39

To run the tests with uvloop installed, it's necessary to use the
``-j`` option to run layers in separate processes.

  default. You must pass a log option specifying a log file name.

Uvloop

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.)

Dont pickle errors

Don't send errors as pickles. Send error names and class dict or arguments.

This is a step toward eliminating instance pickles from the ZEO protocol.

This is much messier than I'd like it to be because of the way both
python and ZEO handle exceptions.

Server sync