low-level data model changes ahead

[Stefan Guggisberg <st...@gmail.com>]

folks!
a while ago while working on concurrency issues we realized that we
need to add an integer modCount field to ItemState and that this
field needs to get persisted as well. the persistence format will
change as a result.

since we haven't released anything yet i guess this is one of the
last occasions for such changes without having to worry about
backwards compatibility issues.

while at it i also plan to clean up some persistence manager
implementations and improve the serialization format currently
used by ObjectPersistenceManager and DbPersistenceManager
(reduce size of binary representation).

i'll commit the changes in the next couple of days unless somebody
raises serious objections.

cheers
stefan