added tests and size limits

broker/src/asapo_broker/database/encoding.go

broker/src/asapo_broker/database/encoding_test.go

broker/src/asapo_broker/database/mongodb.go

consumer/api/cpp/src/consumer_impl.cpp

consumer/api/cpp/unittests/test_consumer_impl.cpp

...atic/mongo_db/insert_retrieve/insert_retrieve_mongodb.cpp

tests/automatic/producer/python_api/producer_api.py