core/gatt: Add UnregisterService and track sender
core/gatt: Add UnregisterService and track sender

This patch implements the GattManager1.UnregisterService method. The
API implementation is slightly changed so that external services are
tracked on a per path + sender basis and not just based on the object
path.

pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,silent-snowman/bluez,silent-snowman/bluez,ComputeCycles/bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,mapfau/bluez,ComputeCycles/bluez,ComputeCycles/bluez,pkarasev3/bluez,pkarasev3/bluez,silent-snowman/bluez,pkarasev3/bluez,ComputeCycles/bluez,mapfau/bluez,mapfau/bluez,mapfau/bluez