Rename one broker call to distinguish it from the other offset calls.

Taking some inspiration from
https://github.com/mumrah/kafka-python/blob/master/kafka/consumer.py

protocol/broker.go

@@ -114,7 +114,7 @@ func (b *Broker) GetMetadata(clientID string, request *MetadataRequest) (*Metada
 	return response, nil
 }
 
-func (b *Broker) GetOffset(clientID string, request *OffsetRequest) (*OffsetResponse, error) {
+func (b *Broker) GetAvailableOffsets(clientID string, request *OffsetRequest) (*OffsetResponse, error) {
 	response := new(OffsetResponse)
 
 	err := b.sendAndReceive(clientID, request, response)