[2108] Make FindResult return non-const ZoneData

This is to make it work with InMemoryClient::add() method, which will
modify existing zone data.

src/lib/datasrc/memory/zone_table.h

@@ -86,11 +86,11 @@ public:
     /// \brief Result data of findZone() method.
     struct FindResult {
         FindResult(result::Result param_code,
-                   const ZoneData* param_zone_data) :
+                   ZoneData* param_zone_data) :
             code(param_code), zone_data(param_zone_data)
         {}
         const result::Result code;
-        const ZoneData* const zone_data;
+        ZoneData* const zone_data;
     };
 
 private: