MXS-2208 Add init/finish functions for mxb::http

Init/finish needed, as curl otherwise will leak memory.

maxutils/maxbase/src/test/test_http.cc

@@ -160,6 +160,8 @@ int main()
     long start;
     long stop;
 
+    mxb::http::Init init;
+
     start = time_since_epoch_ms();
     rv += test_http();
     stop = time_since_epoch_ms();