mnesia: Cleanup and remove obsolete code

We don't support communicating with such old nodes anyway.
mnesia: Improve checkpoint tests
mnesia: Improve checkpoint activation

Fixed a race where some parts of a transaction could
be added to the checkpoint.

There are probably more races here but this improves the current
testcases.
mnesia: Checkpoint fix

Avoid hanging waiting for other processes on other node
to commit.
Merge branch 'maint'