Improve table versioning.

Improve table versioning.

This patch changes the way table version numbers are stored on disk.
Two version numbers instead of only one are stored in the table header:
The version of the lib that created the table, and the lowest version
number of the lib that can read the table.

As the latter is currently 0, it has no effect so far, i.e. tables
created with previous versions of the library still work fine. However,
with this change future versions of the library can detect from the
on-disk versions and the library version numbers if table and library
are compatible.