proc/proc: Fix OSX hangs in highly parallel programs

`next` would hang in highly parallel programs, causing test flickers and
unexpected behavior. This patch fixes it by examining all stopped
threads whenever Delve gets a notification, instead of just the thread
that caused the stop.

proc/proc.go

@@ -309,10 +309,14 @@ func (dbp *Process) next() (err error) {
 	}
 
 	for {
-		th, err := dbp.trapWait(-1)
+		_, err := dbp.trapWait(-1)
 		if err != nil {
 			return err
 		}
+		for _, th := range dbp.Threads {
+			if !th.Stopped() {
+				continue
+			}
 			tg, err := th.GetG()
 			if err != nil {
 				return err
@@ -333,6 +337,7 @@ func (dbp *Process) next() (err error) {
 				return err
 			}
 		}
+	}
 }
 
 func (dbp *Process) setChanRecvBreakpoints() (int, error) {