fixed issues in PreUpdateEventArgs due to different changeset structure

fix https://github.com/doctrine/phpcr-odm-documentation/issues/60

this is a rather large refactoring, undoing the copy of the 'originalData' in the changeset structure that added in the development of 1.2.0 to handle recomputing changesets. this data structure is no longer necessary since we changed the changeset structure to match that of the ORM in order to properly support ``PreUpdateEventArgs`` which already contains the data from the changed fields.

there is still an open question about the behavior of the ORM:
https://github.com/doctrine/doctrine2/commit/bc6714c2c8d015cb2e3ae198fcb1b98b3bbe35e0#commitcomment-7329820
we should probably do the same as orm here, to keep confusion low