Changed index separator to Missing Value sentinel.

This change adds the missing value to the end of the contribution
of each column when generating index keys. This allows us to
reconstruct index keys without reading the source rows.
Tidied code and bumped index metadata version.
Fixed key translation for multivalue columns.
Added extra tests for multivalue columns.
First pass implemenation of index key sentinel.
Added num_rows control to high level tests.
Tidied the implementation of sentinels.
Merge branch 'key_iterator'
Added address_size to XML and bumped versions.