[ZEO4] server: Always iterate over snapshot of .connections[storage_id]

Or else such iteration can race with deletion of elements in that list
on client disconnection. StorageServer.close was already iteration over
snapshot, but StorageServer.invalidate was not.

As the result sending out invalidations were subject to race wrt client
disconnections, and some invalidations could be skipped to be delivered
causing further data corruption.

Fixes Bug2 from https://github.com/zopefoundation/ZEO/issues/209

src/ZEO/StorageServer.py

@@ -1105,7 +1105,7 @@ class StorageServer:
         else:
             assert info is not None
 
-        for p in self.connections[storage_id]:
+        for p in self.connections[storage_id][:]:
             try:
                 if invalidated is not None and p is not conn:
                     p.client.invalidateTransaction(tid, invalidated)

[Kirill Smelkov]

Applied to 4-nxd as 22b2c7a3.