Fix outdated context manager.
Remove debug code from _execute_sql_query.
Merge pull request #114 from astaric/fix-setting-handler

Fix outdated context manager.
Approximate length estimation for SqlTable.
Change _execute_sql_query to a context manager.

It now also uses a connection pool instead of a single connection to
allow background queries performed in parallel.
Do not allow filters with no conditions.
Avoid creating empty filters.
Use approximate data set sizes in Select Rows widget.
Update Data Info with exact size once it is computed.
Do not apply row filters twice.
Always use the same approximation for length.
Merge pull request #112 from lanzagar/approx

Approximate queries
Show tables and views from all schemas.
Integer columns should be either Continuous or Discrete variables.
Fix discrete attributes with int values in database.
Fix a failing test.
Support more sql types.
Remove print line.
Do not include NULLs in distributions.
Do not count NULLS in contingencies.