PROTON-2843: [Go] Fix segfault with acknowledging a message on closed receiver

[PatrickTaibel]

Deliveries can not be settled after the receiver has been freed. The current Go implementation leads to a segmentation fault when the receiver is closed before a delivery is acknowledged.

Therefore, this adds a check to ensure that the receiver was not closed (which ensures that Free has not been called on the link yet). Additionally, this changes to use InjectWait in order to be able to use the newly returned error.