added keys on tx_flux_column and tx_flux_parent

this changed will introduce database keys on tx_flux_column and tx_flux_parent. 

While profiling a TYPO3 6.2 dev installation, we discovered quite a lot of queries for flux without using any keys:

  SELECT * FROM tt_content WHERE ((tx_flux_column = 'S')
  OR (tx_flux_parent = 'S' AND (tx_flux_column = 'S' OR tx_flux_column = 'S')))
  AND deleted = N AND hidden = N GROUP BY uid ORDER BY sorting ASC LIMIT N,N

This change introduces the missing keys. The result ist a faster query execution and the ability to store the result in query cache