Query cancellation

This commit adds query cancelling to the generic sql driver, Druid and
Presto. It cancels the future the query is running in which will
interrupt the query and allow any query cancellation logic to run. If
the database doesn't support query cancellation, or the necessary
information (i.e. query id) isn't available, the code will behave as
it did before.

Switch sql templates to an instaparse grammar [ci drivers]

This commit switches the SQL template parsing to a formally specified
EBNF grammar using instaparse. By specifying it in this way, changes
to the language become more flexible and explicit. This commit also
adds support for multiple parameters specified in an optional clause
as it's baked into the grammar.

Fixes #5492

Search widget [WIP]

Fix the transient failing api.dataset-test [ci drivers]