Record is expected to contain the TID of previous transaction which
modified given object. This is not the meaning of 3rd value returned by
_load (the TID of the next transaction modifying object).
So, instead, cache the last TID of each object, until there is no next
transaction modifying them.*
As this can become a memory problem with big number of objects, add a TODO.

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2262 71dcc9de-d417-0410-9af5-da40c76e7ee4