Fixed some other merge issues with the sorting order of equally timestamped
entries in undo.data (see #1104).
1. The fact that both files can begin with equal timestamps but different
modifications has not been taken into account.
2. Besides the fact that the relative order within the same data file must
be preservered as introduced before, we also need a unique order for entries
of different data files so that each machine comes to the same merge result.
This has now been achieved by taking the UUIDs into account as soon as the
timestamps are equal.
Fixed the bug where the sort order of transactions with equal timestamps
was not stable, i.e. due to the lack of a total order, different merges
produced different sortings and hence messed up transactions which
have already been merged.
Added unit test.
Implemented a quick fix for the issue with a broken branch-off algorithm
which disregarded the transaction boundaries in the undo.data file.
