async: add context support

In order to handle async operations, we need some kind of
'run-loop' around the sd_bus_wait/sd_bus_process process.
Define a 'context' to hold the bus and manage the run-loop.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I16099dd2e15809776827dd85327d950af3f8f5b2
diff --git a/meson.build b/meson.build
index 0a214e2..640059b 100644
--- a/meson.build
+++ b/meson.build
@@ -24,8 +24,9 @@
 root_inc = include_directories('include')
 
 libsdbusplus_src = files(
-    'src/exception.cpp',
+    'src/async/context.cpp',
     'src/bus.cpp',
+    'src/exception.cpp',
     'src/message/native_types.cpp',
     'src/sdbus.cpp',
     'src/server/interface.cpp',