Various: When we opened a new connection before checking BTrees, we also

created the possibility that the checking code would need to try to read
up data from a closed connection.  This turned out to be a frequent source
of new failures in checkConcurrentUpdates1Storage (both FileStorage and BDB
flavors).  Changed all of the relevant tests to grab the BTree anew from
the newly-opened connection.

src/ZEO/tests/InvalidationTests.py

@@ -465,6 +465,7 @@ class InvalidationTests:
             db2._storage.sync()
 
         cn = db1.open()
+        tree = cn.root()["tree"]
         self._check_tree(cn, tree)
         self._check_threads(tree, t1, t2)
 
@@ -489,6 +490,7 @@ class InvalidationTests:
         self.go(stop, cd, t1, t2)
 
         cn = db1.open()
+        tree = cn.root()["tree"]
         self._check_tree(cn, tree)
         self._check_threads(tree, t1, t2)
 
@@ -521,7 +523,9 @@ class InvalidationTests:
             db1._storage.sync()
             db2._storage.sync()
 
+
         cn = db1.open()
+        tree = cn.root()["tree"]
         self._check_tree(cn, tree)
         self._check_threads(tree, t1, t2, t3)
 
@@ -555,7 +559,9 @@ class InvalidationTests:
             db1._storage.sync()
             db2._storage.sync()
 
+
         cn = db1.open()
+        tree = cn.root()["tree"]
         self._check_tree(cn, tree)
         self._check_threads(tree, t1, t2, t3)
 
@@ -591,7 +597,9 @@ class InvalidationTests:
         while db1.lastTransaction() != db2.lastTransaction():
             db1._storage.sync()
             db2._storage.sync()
+
         cn = db1.open()
+        tree = cn.root()["tree"]
         self._check_tree(cn, tree)
 
         # Purge the tree of the dummy entries mapping to 0.