async: add scope

In order to be able to create multiple subtasks as a collection
and in order to make sure we keep good track of any tasks that
are created by the context, add a `scope` as a container of subtasks.
This is partially modelled after the C++ P2519 proposal.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I6e99b2fa2829d80c320491991f7e038f122963a9
diff --git a/meson.build b/meson.build
index af8cdf3..448d167 100644
--- a/meson.build
+++ b/meson.build
@@ -26,6 +26,7 @@
 libsdbusplus_src = files(
     'src/async/context.cpp',
     'src/async/match.cpp',
+    'src/async/scope.cpp',
     'src/bus.cpp',
     'src/event.cpp',
     'src/exception.cpp',