event/DeferredMonitor: make fully thread-safe

Instead of creating a new eventfd for each DeferredMonitor instance,
reuse EventLoop's eventfd, and add a std::list to EventLoop that
manages the list of pending DeferredMonitors.  This std::list is
protected by the same mutex as the "calls" list.

The bottom line is: reduced overhead because the per-instance eventfd
was eliminated, slightly added overhead due to Mutex usage (but
negligible), and we're thread-safe now.

This subsystem is now good enough to replace EventLoop::AddCall().

src/event/DeferredMonitor.cxx

@@ -25,7 +25,7 @@ void
 DeferredMonitor::Cancel()
 {
 #ifdef USE_INTERNAL_EVENTLOOP
-	pending = false;
+	loop.RemoveDeferred(*this);
 #endif
 #ifdef USE_GLIB_EVENTLOOP
 	const auto id = source_id.exchange(0);
@@ -38,8 +38,7 @@ void
 DeferredMonitor::Schedule()
 {
 #ifdef USE_INTERNAL_EVENTLOOP
-	if (!pending.exchange(true))
-		fd.Write();
+	loop.AddDeferred(*this);
 #endif
 #ifdef USE_GLIB_EVENTLOOP
 	const unsigned id = loop.AddIdle(Callback, this);
@@ -49,21 +48,6 @@ DeferredMonitor::Schedule()
 #endif
 }
 
-#ifdef USE_INTERNAL_EVENTLOOP
-
-bool
-DeferredMonitor::OnSocketReady(unsigned)
-{
-	fd.Read();
-
-	if (pending.exchange(false))
-		RunDeferred();
-
-	return true;
-}
-
-#endif
-
 #ifdef USE_GLIB_EVENTLOOP
 
 void