When the database impersonation flag is on, a query using a template
fails. It has to do with templating using a database connection without
a username being specified by the caller, along with the fact that the
work is taking place on a worker, outside a web request, where
referencing g.user raises this exception.
* Add "Impersonate user" setting to Datasource
* Add tests
* Use g.user.username for all the sync cases
* use uri.username instead of uri.user
* Small refactoring
* [explore] fixed padding bug on filter section
* fix slice_id logging issue
* [superset-sqllab] fix slice_id population in appropriate column
* [explore-logging] test the slice_id logging fix
* fix travis errors
* fix nits pointed out in PR comments
* cleanup tests
* made python more beautiful
* made python even more beautiful
* made python even more more beautiful
* made python even more more more beautiful
* fix lint error
* make exception handling more specific
* fixed silly error
* fixed argument indentation
* [add] Save filters to dashboard
* format code
* fix CI error
* add semicolon semi
* fix none object
* add test data
optimize the js code
fix the compatibility issue
* fix urllib to urllib.parse
* add space
* update test case
* remove 'return'
* fix error
* update test case
* upgrade celery to 4.0.2
* using Redis for unit tests (sqla broker not supported in Celery 4)
* Setting Celery's soft_time_limit based on `SQLLAB_ASYNC_TIME_LIMIT_SEC` config
* Better error handling in async tasks
* Better statsd logging in async tasks
* show [pending/running] query status in Results tab
* systematically using sqla NullPool on worker (async) to limit number
of database connections
