asio: unpack in place

The existing code uses several different tuples to do the unpack, then
concatenates them with tuple_cat.  Simplify the code by generating a
tuple of references to the dbus args, and unpack directly to the main
struct, thus simplifying the flow.

To accomplish this the make_dbus_args_tuple helper method is added,
which strips the non-dbus arguments from a callback so they can be
unpacked separately.  This is relatively complex because this code tries
to support

callback(boost::error_code, message_t, dbus_arg);
as well as
callback(boost::error_code, dbus_arg);

Dynamically dependent on which overload is provided.  There's likely
more cleanup that can happen there to separate these two, but this
should at least help.

Tested: OpenBMC launches and runs.  Redfish service validator (which
should hit the majority of these overloads) passes.

Change-Id: I6bb7ee960459a505eb8633b1cdb30cd2b3037466
Signed-off-by: Ed Tanous <etanous@nvidia.com>
diff --git a/test/meson.build b/test/meson.build
index 300c3bc..2b7bcfc 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -37,6 +37,7 @@
     'message/types',
     'timer',
     'unpack_properties',
+    'utility/make_dbus_args_tuple',
     'utility/tuple_to_array',
     'utility/type_traits',
 ]