[SERVICES-2668] Rest endpoints for managing indexing sessions

[mad2sm0key]

Reasoning

• indexing analytics is a long-running task; provide a mechanism for triggering and managing indexing through rest endpoints

Proposed Changes

• add IndexerSession schema to be persisted in mongoDB. A session can contain multiple indexing jobs (1 week intervals)
• add controller with rest endpoints for managing indexing sessions; for simplicity, a limitation of a single active indexing session at a time is imposed
• expose the analytics indexer controller in the private app
• NOTE: indesing sessions are meant to be state machines. Their state will be updated by a cron-job (future PR)

How to test

• GET /analytics-indexer/sessions should return a list of all the indexing sessions
• GET /analytics-indexer/sessions/:nameOrID should return a single indexing session
• POST /analytics-indexer/sessions for creating a new indexing session
• POST /analytics-indexer/sessions/:nameOrID/abort for aborting an active session