Pass and use time_point in DecayingSample ctor (RIPD-359)

Switches a number of places in Resource::Logic to use abstract_clock::now()
which returns a time_point.  Unfortunately Resource::Logic tracks time
locally also, but with ints, not time_point.  So Resource::Logic uses a
delicate mix of abstract_clock::now() and abstract_clock::elapsed() with
this commit.  That inconsistency could be addressed in a second commit.

src/ripple/resource/impl/Key.h

@@ -30,6 +30,26 @@ struct Key
     beast::IP::Endpoint address;
     std::string name;
 
+    // No default constructor
+    Key () = delete;
+
+    // Convenience constructors
+    Key (Kind k, beast::IP::Endpoint const& addr)
+        : kind(k)
+        , address(addr)
+        , name()
+    {
+        assert(kind != kindAdmin);
+    }
+
+    Key (Kind k, const std::string& n)
+        : kind(k)
+        , address()
+        , name(n)
+    {
+        assert(kind == kindAdmin);
+    }
+
     struct hasher
     {
         std::size_t operator() (Key const& v) const