Elevate log level of publishing endpoints.
Fix ingestor shutdown procedure.
Add first/last event timestamp to segment meta data.
Save mode switches to history.

(Plus a few touch-ups.)
Allow for generation of coverage mask.
Fix typos.
Rewrite query processing at the index.

This commit introduces a large change to the query processing architecture of
the index:

    - The query cache moved from SEARCH to INDEX. SEARCH only performs
      bookeeping tasks for now and manages client connections.

    - We have a new algorithm to evaluate queries at the index, which supports
      fine-grained progress indication as well as partial result extraction.
      Each predicate of a query now has associated a list of constraints from
      which we derive the set of affected partitions.

    - INDEX keeps track of the actors which issued a query and sends out push
      notifications whenever the result changed during the distributed lookup
      phase.