[mms-handler-dbus] Made D-Bus calls to MMS handler asynchronous

This eliminates any chance of an inter-process deadlock in case
if MMS handler and the caller of mms-engine is the same component
(which is the case with commhistoryd).
Merge pull request #55 from monich/mms-handler-dbus-async

Made D-Bus calls to MMS handler asynchronous