Previously, if a user changed the query between
requests, the previous query would not be
canceled. This handles that edge-case.

Depending on context in the code, rename cacheKey
to exprID when it identifies a unique expression,
and rename cacheKey to requestID when it is
identifying a request.

Allow the datasource to decide how to handle a
canceled request.

Responsibility is now to pass the cacheKey through
to `datasourceRequest` in each datasources'
implementation of `query`.

Repeat Prometheus requests with the same query
will cancel preceding requests.

Repeat requests, if they already exist in the
in-flight request map, will cause the previous
request to cancel.

The implementation of the unique key is the
responsibility of individual datasources.

Enable the "limit" param in /api/search

Conflicts:
	CHANGELOG.md

Commit adds a check whether Organization exists before User is added to
the organization.

Fixes #3151.

restart grafana after upgrade

I've forgotten to update the systemctl start to restart in my first commit