pvac::Monitor::cancel() don't clear data members

Clearing root, overrun, and changed makes it difficult
to avoid a race during an async. cancel (very common).

src/client/clientMonitor.cpp

@@ -196,9 +196,6 @@ std::string Monitor::name() const
 
 void Monitor::cancel()
 {
-    changed.clear();
-    overrun.clear();
-    root.reset();
     if(impl) impl->cancel();
 }
 

src/client/pva/client.h

@@ -124,7 +124,11 @@ struct epicsShareClass Monitor
     //! Channel name
     std::string name() const;
     //! Immediate cancellation.
-    //! Does not wait for remote confirmation.
+    /** Does not wait for remote confirmation.
+     *
+     @since Up to 7.0.0 also cleared root, changed, and overrun.
+            After 7.0.0, these data members are left unchanged.
+     **/
     void cancel();
     /** updates root, changed, overrun
      *