calls.  If multiple threads sharing a ZEO connection want to make
overlapping calls, they can do that now.  This is mostly useful when
one thread is waiting for a long-running pack() or undo*() call -- the
other thread can now proceed.

Jeremy & I did a review of the StorageServer code and found no place
where overlapping incoming calls from the same connection could do any
harm -- given that the only places where incoming calls can be handled
are those places where the server makes a callback to the client.