common: a simple API to extract md_config_cacher_t cached value

Using a type-deduced operator() (i.e. my_opt() ), instead of the
existing ValueT() operator (which requires a type-matching cast).

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>

src/common/config_cacher.h

@@ -53,6 +53,10 @@ public:
   operator ValueT() const {
     return value_cache.load();
   }
+
+  ValueT operator*() const {
+    return value_cache.load();
+  }
 };
 
 #endif // CEPH_CONFIG_CACHER_H