Add 'aggregates' API endpoint (aggregates/measures distinguishing)

[Asu4ni]

Since in the document 'aggregates' have been distinguished from 'measures', the api should be slightly modified as well.
And maybe at the same time, due to the issue #307, the DELETE methed for 'aggregates' could be implemented as well.

Here is the todo list:

• deprecate GET /v1/metric/<id>/measures
• deprecate GET /v1/batch/metrics/measures
• add GET /v1/metric/<id>/aggregates
• add GET /v1/batch/metrics/aggregates
• modify document to mention the changes
• add release note of the additions and the deprecations

Here is the maybe one:

• change the key value in content returned by 'groupby metric search' from measures to aggregates
• add DELETE /v1/metric/<id>/aggregates to delete the processed aggregates. The range to delete can be limited by start and end parameter
• change names in the code to aggregates if needed

If there are something missed or to be modified, please comment to let me know.