Give access to previous versions' stores in upgrade() functions

[dfahlander]

In case the previous version's store was removed, and the upgrade function needs to copy items from it, it wont have access to the old store to do that. The use case is that an architecture has changed and objects from one store needs to be copied to another or splitted to several other stores.

There is also a bug in IE that prohibits reading from an object store before deleting the store. If fixing this enhancement we would also need to work around that IE bug by closing and reopening the database towards an intermediate version that would delete the old stores.