IPC: make each connection ref the owning service

This is because the connection functions use the c->service pointer
and this needs to be mirrored in the reference counting.

The service can only be free'd when all connections are destroyed
and the user as unreferenced all previously referenced connections
and the service.

Fixes #62
Thanks to Jan Friesse for the reproducer
Signed-off-by: Angus Salkeld <asalkeld@redhat.com>