* make TimeRange interface and add relative range
* make Execute methods support the current time
* update resample to support relative time range
* update DSNode to support relative time range
* update query service to create queries with absolute time
* make alerting evaluator create relative time ranges
This commit fixes a bug where queries from datasources such as InfluxDB that returned no data would not create a DatasourceNoData alert, but instead an error "can only reduce type series, got type noData".