Insert fake events under a lock

The thread-specific spinlock needs to be acquired before a fake event is
inserted from a non-polling thread. The usual situation is when a monitor
thread inserts a hangup event for a DCB.

Other, less common cases are when session timeouts have been enabled and
the DCB needs to be closed. Here it is better to insert a fake hangup
event instead of directly closing the DCB from an external thread.

server/core/session.c

@@ -42,6 +42,7 @@
 #include <maxscale/atomic.h>
 #include <maxscale/log_manager.h>
 #include <maxscale/housekeeper.h>
+#include <maxscale/poll.h>
 
 /* This list of all sessions */
 LIST_CONFIG SESSIONlist =
@@ -927,7 +928,7 @@ void process_idle_sessions()
                 if (all_session->service && all_session->client_dcb && all_session->client_dcb->state == DCB_STATE_POLLING &&
                     hkheartbeat - all_session->client_dcb->last_read > all_session->service->conn_idle_timeout * 10)
                 {
-                    dcb_close(all_session->client_dcb);
+                    poll_fake_hangup_event(all_session->client_dcb);
                 }
 
                 current = list_iterate(&SESSIONlist, current);