LibIPC: Use a zero-delay timer for message processing

This lets us avoid using Core::deferred_invoke() which is not usable
during application teardown (as there is no event loop to push the
deferred invocation onto.)

(Not that there is an event loop to fire the processing timer during
teardown *either*, but at least we can exit gracefully with pending
timers, unlike deferred invocations, which hang the process. This is an
area where more improvements are definitely needed!)

Userland/Libraries/LibIPC/Connection.cpp

@@ -17,6 +17,7 @@ ConnectionBase::ConnectionBase(IPC::Stub& local_stub, NonnullRefPtr<Core::LocalS
     , m_local_endpoint_magic(local_endpoint_magic)
 {
     m_responsiveness_timer = Core::Timer::create_single_shot(3000, [this] { may_have_become_unresponsive(); });
+    m_processing_timer = Core::Timer::create_single_shot(0, [this] { handle_messages(); });
 }
 
 ConnectionBase::~ConnectionBase()
@@ -175,9 +176,8 @@ bool ConnectionBase::drain_messages_from_peer()
     }
 
     if (!m_unprocessed_messages.is_empty()) {
-        deferred_invoke([this] {
-            handle_messages();
-        });
+        if (!m_processing_timer->is_active())
+            m_processing_timer->start();
     }
     return true;
 }

Userland/Libraries/LibIPC/Connection.h

@@ -60,6 +60,7 @@ protected:
 
     NonnullRefPtr<Core::LocalSocket> m_socket;
     RefPtr<Core::Timer> m_responsiveness_timer;
+    RefPtr<Core::Timer> m_processing_timer;
 
     RefPtr<Core::Notifier> m_notifier;
     NonnullOwnPtrVector<Message> m_unprocessed_messages;