If a transaction takes a long time to complete after it has the
storage lock, close the connection and abort the transaction.  The
abort may cause the client to be hosed.

XXX There should be a configuration option to enable / disable this
feature.  That should be done before too long.

When a transaction takes a long time to complete, it is often a sign
of deadlock involving multiple storages.  Until the possibility of
deadlock is eliminated, this feature will prevent an entire ZEO
cluster from deadlocking (on writes).