Use a quite faster SQL query, down from 0.02" to something not mesurable.

Help a bit fixing the lighttable slowness when displaying images based
on tag.
Add index on meta_data.

Speed-up the lighttable as we are doing lot of queries on meta_data table.
Merge branch 'lighttable-perfs'

* lighttable-perfs:
  Add index on meta_data.
  Use a quite faster SQL query, down from 0.02" to something not mesurable.