ZEO4 servers can send invalidations out of order with
``getInvalidations`` results, presumably because ``getInvalidations``
didn't get the commit lock.  ZEO4 clients worked around this (maybe
not directly) by queuing invalidations during cache verification.
ZEO5 servers don't send invalidations out of order with
``getInvalidations`` calls and the ZEO5 client didn't need an
invalidation queue, except they do need one to work correctly with
ZEO4 servers. :/