This fixes problems in multi-database setups (ie. ZODB and other
databases).
For example, imagine a transaction stores an object in ZODB and its
identifier in a relational database.
Then, another process connected to the same databases and polling the
relational database find the new entry, and tried to load the object from
ZODB.
If the second node didn't process invalidation sent by initial transaction
when it starts its transaction, second client would not be able to see that
object, which should not happen (inter-database inconsistency).

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2329 71dcc9de-d417-0410-9af5-da40c76e7ee4