Refactor usage of legacy data contracts. Moves legacy data contracts
to pkg/tsdb/legacydata package.
Refactor pkg/expr to be a proper service/dependency that can be provided
to wire to remove some unneeded dependencies to SSE in ngalert and other places.
Refactor pkg/expr to not use the legacydata,RequestHandler and use
backend.QueryDataHandler instead.
* context all the things
* apply feedback
* rollback some alerting changes
* rollback some alerting changes #2
* more rollbacks
* more rollbacks #2
* more rollbacks #3
* more rollbacks #4
* fix integration test
* add missing context
* add missing and remove incorrect dispatch