can cause signidficant delays, especially on systems with very
course-grained sleeps, like most linux systems.  This change makes the
ZEO tests run about 25% faster on an Ubuntu desktop system.  We
suspect the production impact to be much greater, at least on some
systems.

Removed some non-async code, now that we no-longer have a non-async
mode. (I cowardly left an assert behind to make sure.:)