Prevent accidental aggregates

*  The compiler can provide many non-explicit constructors for
   aggregate types.  This is sometimes desired, but it can
   happen accidentally, resulting in run-time errors.

*  This commit assures that no types are aggregates unless existing
   code is using aggregate initialization.

src/ripple/rpc/impl/ServerHandlerImp.h

@@ -46,11 +46,15 @@ class ServerHandlerImp
 public:
     struct Setup
     {
+        explicit Setup() = default;
+
         std::vector<Port> ports;
 
         // Memberspace
         struct client_t
         {
+            explicit client_t() = default;
+
             bool secure = false;
             std::string ip;
             std::uint16_t port = 0;
@@ -66,6 +70,8 @@ public:
         // Configuration for the Overlay
         struct overlay_t
         {
+            explicit overlay_t() = default;
+
             boost::asio::ip::address ip;
             std::uint16_t port = 0;
         };