Review Connection/DB closure

In addition to some micro-optimisation, this fixes the following minor issues:
- Closing a DB left Connections registered to the global transaction manager.
  Which broke at least multi-db because only such primary connections were
  ignored with monkey-patches.
- Stop calling the real newTransaction when aborting an existing transaction.
  This was not the case if explicit_transactions=0.

src/ZODB/Connection.py

@@ -283,7 +283,6 @@ class Connection(ExportImport, object):
             raise ConnectionStateError("Cannot close a connection joined to "
                                        "a transaction")
 
-        if self._cache is not None:
         self._cache.incrgc() # This is a good time to do some GC
 
         # Call the close callbacks.
@@ -734,8 +733,6 @@ class Connection(ExportImport, object):
 
     def newTransaction(self, transaction, sync=True):
         self._readCurrent.clear()
-
-        try:
         self._storage.sync(sync)
         invalidated = self._storage.poll_invalidations()
         if invalidated is None:
@@ -743,8 +740,6 @@ class Connection(ExportImport, object):
             # we need to flush the whole cache.
             invalidated = self._cache.cache_data.copy()
         self._cache.invalidate(invalidated)
-        except AttributeError:
-            assert self._storage is None
 
     def afterCompletion(self, transaction):
         # Note that we we call newTransaction here for 2 reasons:
@@ -924,7 +919,6 @@ class Connection(ExportImport, object):
 
         transaction_manager.registerSynch(self)
 
-        if self._cache is not None:
         self._cache.incrgc() # This is a good time to do some GC
 
         if delegate:
@@ -951,7 +945,7 @@ class Connection(ExportImport, object):
                 c._storage.release()
             c._storage = c._normal_storage = None
             c._cache = PickleCache(self, 0, 0)
-            c.transaction_manager = None
+            c.close(False)
 
     ##########################################################################
     # Python protocol

src/ZODB/DB.py

@@ -24,7 +24,7 @@ from . import utils
 
 from ZODB.broken import find_global
 from ZODB.utils import z64
-from ZODB.Connection import Connection, TransactionMetaData
+from ZODB.Connection import Connection, TransactionMetaData, noop
 from ZODB._compat import Pickler, _protocol, BytesIO
 import ZODB.serialize
 
@@ -646,15 +646,15 @@ class DB(object):
         is closed, so they stop behaving usefully.  Perhaps close()
         should also close all the Connections.
         """
-        noop = lambda *a: None
         self.close = noop
 
         @self._connectionMap
-        def _(c):
-            if c.transaction_manager is not None:
-                c.transaction_manager.abort()
-            c.afterCompletion = c.newTransaction = c.close = noop
-            c._release_resources()
+        def _(conn):
+            if conn.transaction_manager is not None:
+                for c in six.itervalues(conn.connections):
+                    c.explicit_transactions = True
+                conn.transaction_manager.abort()
+            conn._release_resources()
 
         self._mvcc_storage.close()
         del self.storage