Make sure the ZEO 5 client works with the ZEO 4 server

Added an option to test the ZEO 5 client against the ZEO 4
server. This involved making a copy of the ZEO 4 server in
ZEO.tests.ZEO4 and disabling tests of ZEO5-specific features.

Testing revealed bugs in support for ZEO 4 servers, which were fixed.

CHANGES.rst

 Changelog
 =========
 
+- Fixed bugs in using the ZEO 5 client with ZEO 4 servers.
+
 5.0.0a2 (2016-07-30)
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