[buteo-syncfw] Fix leak of plugin service object

This commit ensures that the destructor of the plugin service object
is called explicitly, to ensure proper cleanup.  It also adds checks
to the plugin service object code to ensure that if member functions
are called prior to initialization the plugin emits the appropriate
warning instead of crashing.
Merge pull request #63 from chriadam/serviceobj

[buteo-syncfw] Fix leak of plugin service object