- Stop using a list of (in)validated tid (hence removal of RevisionIndex),
  because it can't work in all cases and would even cause memory leaks.
  For example, this bug could lead to ConflictError with a single client.
  Fixit it also requires that database backends always return the next serial.
- Several performance improvements. The most important one is when the latest
  version of an object is cached: it inherits the access counter of the
  previous one (for the same oid), which gets in turn its counter reset.
- Do not waste CPU evaluating the real size taken by an entry in memory.
  Just use 'len' on the value (which is always a pickle data, i.e. a string).

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2711 71dcc9de-d417-0410-9af5-da40c76e7ee4