Move bmcweb over to sdbusplus

This patchset moves bmcweb from using boost-dbus over entirely to
sdbusplus.  This has some nice improvements in performance (about 30%
of CPU cycles saved in dbus transactions), as well as makes this
project manuver closer to the upstream way of thinking.

Changes to bmcweb are largely ceremonial, and fall into a few
categories:
1. Moves async_method_call instances to the new format, and deletes any
use of the "endpoint" object in leiu of the sdbusplus style interface
2. sdbus object_path object doesn't allow access to the string
directly, so code that uses it moves to explicit casts.
3. The mapbox variant, while attempting to recreate boost::variant,
misses a T* get<T*>() method implementation, which allows using variant
without exceptions.  Currently, there is an overload for
mapbox::get_ptr implementation which replecates the functionality.

Tested by: Booting the bmcweb on a target, iterating through redfish
basic phosphor-webui usage, and websockets usage

Change-Id: I2d95882908d6eb6dba00b9219a221dd96449ca7b
Signed-off-by: Ed Tanous <ed.tanous@intel.com>
diff --git a/include/dbus_singleton.hpp b/include/dbus_singleton.hpp
index e2fd2d6..e6be81b 100644
--- a/include/dbus_singleton.hpp
+++ b/include/dbus_singleton.hpp
@@ -1,10 +1,21 @@
 #pragma once
-#include <dbus/connection.hpp>
+#include <sdbusplus/asio/connection.hpp>
+#include <iostream>
+
+namespace mapbox {
+template <typename T, typename... Types>
+const T* get_ptr(const mapbox::util::variant<Types...>& v) {
+  if (v.template is<std::remove_const_t<T>>()) {
+    return &v.template get_unchecked<std::remove_const_t<T>>();
+  } else {
+    return nullptr;
+  }
+}
+}  // namespace mapbox
 
 namespace crow {
 namespace connections {
+static std::shared_ptr<sdbusplus::asio::connection> system_bus;
 
-static std::shared_ptr<dbus::connection> system_bus;
-
-}  // namespace dbus
-}  // namespace crow
\ No newline at end of file
+}  // namespace connections
+}  // namespace crow