All endpoints but _session support gzip encoding and there's no practical reason for that.

This commit enables gzip decoding on compressed requests to _session.

Add missing default headers to responses

Fix semantics of total_rows

The `total_rows` field suppose to be a number of documents in the database/view.
See https://docs.couchdb.org/en/stable/api/ddoc/views.html.

When new pagination API was introduced the meaning of `total_rows` field was
changed to number of rows in the query results. The PR reverts this
accidental change.

This changes allows administators to disable custom JavaScript reduce
functions. Custom reduce functions are notoriously difficult to write
correctly and most of the time when written correctly would be more
efficiently done using an builtin reduce.