queue_processors: Remove the slow_queries queue.

While this functionality to post slow queries to a Zulip stream was
very useful in the early days of Zulip, when there were only a few
hundred accounts, it's long since been useless since (1) the total
request volume on larger Zulip servers run by Zulip developers, and
(2) other server operators don't want real-time notifications of slow
backend queries.  The right structure for this is just a log file.

We get rid of the queue and replace it with a "zulip.slow_queries"
logger, which will still log to /var/log/zulip/slow_queries.log for
ease of access to this information and propagate to the other logging
handlers.  Reducing the amount of queues is good for lowering zulip's
memory footprint and restart performance, since we run at least one
dedicated queue worker process for each one in most configurations.

zerver/middleware.py

@@ -21,7 +21,6 @@ from zerver.lib.debug import maybe_tracemalloc_listen
 from zerver.lib.db import reset_queries
 from zerver.lib.exceptions import ErrorCode, JsonableError, RateLimited
 from zerver.lib.html_to_text import get_content_description
-from zerver.lib.queue import queue_json_publish
 from zerver.lib.rate_limiter import RateLimitResult
 from zerver.lib.response import json_error, json_response_from_error
 from zerver.lib.subdomains import get_subdomain
@@ -30,6 +29,7 @@ from zerver.lib.types import ViewFuncT
 from zerver.models import Realm, flush_per_request_caches, get_realm
 
 logger = logging.getLogger('zulip.requests')
+slow_query_logger = logging.getLogger('zulip.slow_queries')
 
 def record_request_stop_data(log_data: MutableMapping[str, Any]) -> None:
     log_data['time_stopped'] = time.time()
@@ -210,8 +210,7 @@ def write_log_line(log_data: MutableMapping[str, Any], path: str, method: str, r
         logger.info(logger_line)
 
     if (is_slow_query(time_delta, path)):
-        queue_json_publish("slow_queries", dict(
-            query="%s (%s)" % (logger_line, requestor_for_logs)))
+        slow_query_logger.info(logger_line)
 
     if settings.PROFILE_ALL_REQUESTS:
         log_data["prof"].disable()