Drop unneeded use of thread-local.

memcache module already handle thread-safety, and we actually want to be
transaction-local here, so use a volatile variable instead for caching
needs.
Removes the need to restart Zope for connection parameter changes to take
effect.
Fixes a bad interaction with SelectionTool's code, which causes
transactional isolation to be broken (just for this connector) and too many
network connections to be used.
Fix random failure while updating simulation tree after deletion of root delivery line