[2376] Some forward declarations

So we can include less

src/lib/datasrc/master_loader_callbacks.h

@@ -18,6 +18,10 @@
 #include <dns/master_loader_callbacks.h>
 
 namespace isc {
+namespace dns {
+class Name;
+class RRClass;
+}
 namespace datasrc {
 
 class ZoneUpdater;

src/lib/dns/master_loader_callbacks.h

@@ -15,16 +15,18 @@
 #ifndef MASTER_LOADER_CALLBACKS_H
 #define MASTER_LOADER_CALLBACKS_H
 
-#include <dns/rrset.h>
-
 #include <exceptions/exceptions.h>
 
 #include <string>
 #include <boost/function.hpp>
+#include <boost/shared_ptr.hpp>
 
 namespace isc {
 namespace dns {
 
+class AbstractRRset;
+typedef boost::shared_ptr<AbstractRRset> RRsetPtr;
+
 /// \brief Type of callback to add a RRset.
 ///
 /// This type of callback is used by the loader to report another loaded