Fixed a bug handling ZEO4 invalidations during cache verification

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. :/

src/ZEO/asyncio/client.py

@@ -313,6 +313,12 @@ class Client(object):
         self.protocols = ()
         self.disconnected(None)
 
+        # Work around odd behavior of ZEO4 server. It may send
+        # invalidations for transactions later than the result of
+        # getInvalidations.  While we support ZEO 4 servers, we'll
+        # need to keep an invalidation queue. :(
+        self.verify_invalidation_queue = []
+
     def new_addrs(self, addrs):
         self.addrs = addrs
         if self.trying_to_connect():
@@ -409,6 +415,8 @@ class Client(object):
 
     @future_generator
     def verify(self, server_tid):
+        self.verify_invalidation_queue = [] # See comment in init :(
+
         protocol = self.protocol
         if server_tid is None:
             server_tid = yield protocol.fut('lastTransaction')
@@ -465,6 +473,12 @@ class Client(object):
             self.cache.setLastTid(server_tid)
             self.ready = True
 
+            # Gaaaa, ZEO 4 work around. See comment in __init__. :(
+            for tid, oids in self.verify_invalidation_queue:
+                if tid > server_tid:
+                    self.invalidateTransaction(tid, oids)
+            self.verify_invalidation_queue = []
+
             try:
                 info = yield protocol.fut('get_info')
             except Exception as exc:
@@ -597,6 +611,8 @@ class Client(object):
                 self.cache.invalidate(oid, tid)
             self.client.invalidateTransaction(tid, oids)
             self.cache.setLastTid(tid)
+        else:
+            self.verify_invalidation_queue.append((tid, oids))
 
     def serialnos(self, serials):
         # Method called by ZEO4 storage servers.