PVA client ref. loop breaking

Adjust ownership of BaseRequestImpl sub-classes.
Keep two weak_ptr<>.
one (internal) for tracking relations with Channel.
one (external) for tracking relations w/ user code, and send Q.

external wraps internal, and calls destroy() to break
ref. loops involving internal.

death to m_thisPointer!

src/client/pv/monitor.h

@@ -54,7 +54,7 @@ class epicsShareClass MonitorElement {
  * This is used by pvAccess to implement monitors.
  * @author mrk
  */
-class epicsShareClass Monitor : public epics::pvData::Destroyable{
+class epicsShareClass Monitor : public virtual epics::pvData::Destroyable{
     public:
     POINTER_DEFINITIONS(Monitor);
     virtual ~Monitor(){}

src/client/pv/pvAccess.h

@@ -138,7 +138,7 @@ struct epicsShareClass ChannelBaseRequester : virtual public epics::pvData::Requ
 /**
  * Base interface for all channel requests (aka. Operations).
  */
-class epicsShareClass ChannelRequest : public epics::pvData::Destroyable, public Lockable, private epics::pvData::NoDefaultMethods {
+class epicsShareClass ChannelRequest : public virtual epics::pvData::Destroyable, public Lockable, private epics::pvData::NoDefaultMethods {
 public:
     POINTER_DEFINITIONS(ChannelRequest);