Prefer std::optional over boost:optional:

Some of the boost::optionals must remain for now.  Both
boost::beast and SOCI have interfaces that require
boost::optional.

src/test/rpc/Subscribe_test.cpp

@@ -98,7 +98,7 @@ public:
 
             // Check stream update
             auto jvo = wsc->getMsg(10ms);
-            BEAST_EXPECTS(!jvo, "getMsg: " + to_string(jvo.get()));
+            BEAST_EXPECTS(!jvo, "getMsg: " + to_string(jvo.value()));
         }
     }