Name all dameon threads

It is currently difficult to see what's running in a process
in e.g. `pstree`, since all threads spawned from a `Daemon`
are unnamed and appear as `dameon.rb:23`. I therefore added
a `name` method that has a sane default (snake case class)
and can be overridden if desired.

lib/gitlab/daemon.rb

@@ -28,6 +28,10 @@ module Gitlab
       true
     end
 
+    def thread_name
+      self.class.name.demodulize.underscore
+    end
+
     def start
       return unless enabled?
 
@@ -35,7 +39,10 @@ module Gitlab
         break thread if thread?
 
         if start_working
-          @thread = Thread.new { run_thread }
+          @thread = Thread.new do
+            Thread.current.name = thread_name
+            run_thread
+          end
         end
       end
     end

lib/gitlab/sidekiq_daemon/monitor.rb

@@ -4,6 +4,7 @@ module Gitlab
   module SidekiqDaemon
     class Monitor < Daemon
       include ::Gitlab::Utils::StrongMemoize
+      extend ::Gitlab::Utils::Override
 
       NOTIFICATION_CHANNEL = 'sidekiq:cancel:notifications'
       CANCEL_DEADLINE = 24.hours.seconds
@@ -24,6 +25,11 @@ module Gitlab
         @jobs_mutex = Mutex.new
       end
 
+      override :thread_name
+      def thread_name
+        "job_monitor"
+      end
+
       def within_job(worker_class, jid, queue)
         jobs_mutex.synchronize do
           jobs[jid] = { worker_class: worker_class, thread: Thread.current, started_at: Gitlab::Metrics::System.monotonic_time }