added negative acks

CHANGELOG.md

broker/src/asapo_broker/database/mongodb.go

broker/src/asapo_broker/database/mongodb_test.go

broker/src/asapo_broker/server/get_commands_test.go

broker/src/asapo_broker/server/get_next.go

broker/src/asapo_broker/server/post_op_image.go

broker/src/asapo_broker/server/post_op_image_test.go

broker/src/asapo_broker/server/server.go

broker/src/asapo_broker/server/server_nottested.go

consumer/api/cpp/include/consumer/data_broker.h

consumer/api/cpp/src/server_data_broker.cpp

consumer/api/cpp/src/server_data_broker.h

consumer/api/cpp/unittests/test_server_broker.cpp

consumer/api/python/asapo_consumer.pxd

consumer/api/python/asapo_consumer.pyx.in

tests/automatic/consumer/consumer_api/consumer_api.cpp

tests/automatic/consumer/consumer_api_python/consumer_api.py