===================
- Support only named parameters (e.g.: ``:param``) in queries (21, 24).
- Add JSON-schema validation for config file (23).
- Validate at startup if database engines from DSNs are supported and
corresponding modules are available.
- Check that names for queries ``parameters`` match the ones in queries SQL.
- Add ``--check-only`` command line option to just validate configuration.
- Drop support for matching query columns positionally, only support name
match. This is to avoid confusing behavior with positional match, and make
queries more explicit.
**NOTE**:
some of the changes above for query definitions are backwards incompatible,
thus queries might need updating. Specifically:
- Only named parameters with the ``:param`` style are now supported, queries
using positional parameters or other styles of named parameters need to be
updated.
- Literal ``:`` at the beginning of a word need to be escaped (with backslash)
to avoid confusion with parameter markers. Colons that appear inside words
don't need to be escaped.
- Column names for query results must now always match metric and label names
involved in the query. Position-based match for queries without labels is no
longer supported. Queries can be updated adding ``AS
<metric_name|label_name>`` expressions.