http: enclose libevent-dependent code in a namespace

This commit is a no-op to isolate HTTP methods and objects that
depend on libevent. Following commits will add replacement objects
and methods in a new namespace for testing and review before
switching over the server.

src/httprpc.cpp

@@ -26,6 +26,8 @@
 #include <string>
 #include <vector>
 
+using http_libevent::EventBase;
+using http_libevent::HTTPRequest;
 using util::SplitString;
 using util::TrimStringView;