event/Loop: add method SetThread()

Require to call it before calling Run(); remove the setter from Run().
This gives class EventThread more control over when it is initialized.

src/event/Loop.cxx

@@ -269,8 +269,7 @@ void
 EventLoop::Run() noexcept
 {
 #ifdef HAVE_THREADED_EVENT_LOOP
-	if (thread.IsNull())
-		thread = ThreadId::GetCurrent();
+	assert(!thread.IsNull());
 #endif
 
 	assert(IsInside());

src/event/Loop.hxx

@@ -147,6 +147,12 @@ public:
 	explicit EventLoop(ThreadId _thread);
 
 	EventLoop():EventLoop(ThreadId::GetCurrent()) {}
+
+	void SetThread(ThreadId _thread) noexcept {
+		assert(thread.IsNull());
+
+		thread = _thread;
+	}
 #else
 	EventLoop();
 #endif

src/event/Thread.cxx

@@ -39,6 +39,8 @@ EventThread::Run() noexcept
 {
 	SetThreadName(realtime ? "rtio" : "io");
 
+	event_loop.SetThread(ThreadId::GetCurrent());
+
 	if (realtime) {
 		SetThreadTimerSlack(std::chrono::microseconds(10));