MEDIUM: tasklets: Make the tasklet list a struct mt_list.

Change the tasklet code so that the tasklet list is now a mt_list.
That means that tasklet now do have an associated tid, for the thread it
is expected to run on, and any thread can now call tasklet_wakeup() for
that tasklet.
One can change the associated tid with tasklet_set_tid().
diff --git a/src/checks.c b/src/checks.c
index b879100..6bde412 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -1638,6 +1638,8 @@
 	conn = cs->conn;
 	/* Maybe there were an older connection we were waiting on */
 	check->wait_list.events = 0;
+	tasklet_set_tid(check->wait_list.tasklet, tid);
+
 
 	if (!sockaddr_alloc(&conn->dst))
 		return SF_ERR_RESOURCE;
@@ -2891,6 +2893,8 @@
 				cs_destroy(check->cs);
 			}
 
+			tasklet_set_tid(check->wait_list.tasklet, tid);
+
 			check->cs = cs;
 			conn = cs->conn;
 			/* Maybe there were an older connection we were waiting on */