For a flexible attribute query, replace the `col_name` property with
a function call that extracts that attribute from the `field_attrs`
field introduced in the earlier commit.
Additionally, for boolean, numeric and date queries CAST the value to
NUMERIC SQLite affinity to ensure that our queries like 'flex:1..5' and
'flex:true' continue working fine.
This removes the concept of 'slow query', since every query for any
field now has an SQL clause.
