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/test/httpserver_tests.cpp

@@ -12,6 +12,7 @@ BOOST_FIXTURE_TEST_SUITE(httpserver_tests, BasicTestingSetup)
 
 BOOST_AUTO_TEST_CASE(test_query_parameters)
 {
+    using http_libevent::GetQueryParameterFromUri;
     std::string uri {};
 
     // No parameters