setUp():  Use a much smaller (than default) ZEO client cache.

The current default is 200MB, and at least testSerialization reads
the entire cache file into one giant string.  That grossly boosts
the process highwater mark when running the tests.  Windows reclaims
the memory after, but other platforms may not.

Cool:  testSerialization was one of the slowest tests before, but
is 100x faster now.