This fixes a regression introduced by implementation of IMVCCStorage in r2532.
On recent ZODB, this fixes a severe performance issue.
With ZODB 3.4, objects were never invalidated, which was even worse.

This fixes includes compatibility code for ZODB 3.4 so that each connection
has its own NEOStorage instance.
The DB's storage is changed to always consider the last revision of
objects.

IMVCCStorage seems too complicated to me. Connection should be a better
place to implement it (by extending/fixing the 'before' attribute).
So we have decided that NEOStorage stops implementing IMVCCStorage.

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2802 71dcc9de-d417-0410-9af5-da40c76e7ee4