* add test for the bug
* update backtesting evaluators to accept a number of evaluations instead of `to` to have control over the number evaluations in one place
This commit adds support for concurrent queries when saving alert
instances to the database. This is an experimental feature in
response to some customers experiencing delays between rule evaluation
and sending alerts to Alertmanager, resulting in flapping. It is
disabled by default.
* Implement backtesting engine that can process regular rule specification (with queries to datasource) as well as special kind of rules that have data frame instead of query.
* declare a new API endpoint and model
* add feature toggle `alertingBacktesting`
