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.

tools/test-queue-worker-reload

@@ -14,7 +14,7 @@ sanity_check.check_venv(__file__)
 
 # TODO: Convert this to use scripts/lib/queue_workers.py
 TOOLS_DIR = os.path.dirname(os.path.abspath(__file__))
-successful_worker_launch = '[process_queue] 17 queue worker threads were launched\n'
+successful_worker_launch = '[process_queue] 16 queue worker threads were launched\n'
 
 def check_worker_launch(run_dev: "subprocess.Popen[str]") -> bool:
     failed = False