This allows parallel execution of tpc_begin, stores & related conflict
resolution and tpc_vote for different transactions.
This requires an extension to ZODB allowing to keep TID secret until
tpc_finish (ie, so that it doesn't require tpc_vote to return tid for each
stored object).

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2534 71dcc9de-d417-0410-9af5-da40c76e7ee4