api cleanup:

[adamstruck]

• removed dedicated aggregation endpoint
• ListIndices and ListGraphs are no longer streaming endpoints
• adds a ListLabels endpoint
  • this required indexing edges by label in the KVStore

[kellrott]

Have you done any testing to see the speed difference between the mongo queries generated using the specialized aggregation endpoint and the more generic one in this PR? Does it slow down per-label aggregations?

[adamstruck]

I haven't tested it, but for mongo it shouldn't make too much of a difference since under the both use the mono pipeline engine. Elasticsearch aggregation performance will be impacted by this change.

[kellrott]

👍