A boolean type makes the SQLite dialect produce a costruct like
'func() = 1' in WHERE condition. While syntactically correct, it tends
to confuse the query planer.
Transfer is_address_point into SQLAlchemy function, so that
json.has_key() can use the older json_extract() function.
And work around broken Distance function.
When expressions are generated with SQLAlchemy, any constants are
replaced with bind parameters. The bind parameters become parameters of
prepared statements. The result is that the query planner tends to
oversee that the partial indexes can be used.
