ClientStorage could be in either "sync" mode or "async" mode.  Now
there is just "async" mode.  There is now a dedicicated asyncore main
loop dedicated to ZEO clients.

This addresses a test failure on Mac OS X,
http://www.zope.org/Collectors/Zope3-dev/650, that I believe was due
to a bug in sync mode. Some asyncore-based code was being called from
multiple threads that didn't expect to be.

Converting to always-async mode revealed some bugs that weren't caught
before because the tests ran in sync mode.  These problems could
explain some problems we've seen at times with clients taking a long
time to reconnect after a disconnect.

Added a partial heart beat to try to detect lost connections that
aren't otherwise caught,
http://mail.zope.org/pipermail/zodb-dev/2005-June/008951.html, by
perioidically writing to all connections during periods of inactivity.