tornado: Limit the width of the user queries, when they're needed.

Tornado requests try hard to not make SQL queries -- and if they're
necessary, to minimize the number of them.  Specifically, both session
objects and user objects are cached in memcached, and we expect that
both of them will have been filled there by Django before any requests
are made to Tornado.

In the event that memcached is flushed, or data is otherwise evicted,
we perform two database queries -- one for the session, and one for
the user.  However, the *width* of the latter query has grown
significantly over time, as the Realm object grew more fields, and
recently with the addition of role groups, which require multiple
joins each.  This leads to a query which is over 12k of text long, and
results in 319 columns.  In the event of a memcached flush, this can
result in a *significant* amount of SQL traffic, as nearly every
active Tornado request will make that query.

We do not wish to narrow the default query for Django; we instead tag
the request in the REST wrapper, and use that to use a much narrower
user cache entry.  That narrower cache entry is filled before the
queue is created in Django; we also use it to explicitly set the log
data, so the second "half" of the continued Tornado request does not
need to fetch any user data either when writing its log line.

Because they use different cache keys, this only affects the
session-based `/json/events` endpoint, which caches by user-id; the
`/api/v1/events` endpoint, which uses an API-key cache, keeps its wide
user object.  The former is 50% of the total request volume, whereas
the latter is only 2%, so adding an additional cache for it is
unnecessary complexity.

zproject/urls.py

@@ -523,7 +523,11 @@ v1_api_and_json_patterns = [
     # used to register for an event queue in tornado
     rest_path("register", POST=(events_register_backend, {"allow_anonymous_user_web"})),
     # events -> zerver.tornado.views
-    rest_path("events", GET=get_events, DELETE=cleanup_event_queue),
+    rest_path(
+        "events",
+        GET=(get_events, {"narrow_user_session_cache"}),
+        DELETE=(cleanup_event_queue, {"narrow_user_session_cache"}),
+    ),
     # Used to generate a Zoom video call URL
     rest_path("calls/zoom/create", POST=make_zoom_video_call),
     # Used to generate a BigBlueButton video call URL