Add wait API endpoint for waiting on process exit

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

linux/shim/exec_state.go

@@ -46,6 +46,9 @@ func (s *execCreatedState) Start(ctx context.Context) error {
 func (s *execCreatedState) Delete(ctx context.Context) error {
 	s.p.mu.Lock()
 	defer s.p.mu.Unlock()
+	if err := s.p.delete(ctx); err != nil {
+		return err
+	}
 	return s.transition("deleted")
 }
 
@@ -151,6 +154,9 @@ func (s *execStoppedState) Start(ctx context.Context) error {
 func (s *execStoppedState) Delete(ctx context.Context) error {
 	s.p.mu.Lock()
 	defer s.p.mu.Unlock()
+	if err := s.p.delete(ctx); err != nil {
+		return err
+	}
 	return s.transition("deleted")
 }