refactor broker

broker/src/asapo_broker/database/database.go

broker/src/asapo_broker/database/database_test.go

broker/src/asapo_broker/database/mock_database.go

broker/src/asapo_broker/database/mongodb.go

broker/src/asapo_broker/database/mongodb_test.go

broker/src/asapo_broker/server/get_id.go

broker/src/asapo_broker/server/get_id_test.go

broker/src/asapo_broker/server/get_last.go

broker/src/asapo_broker/server/get_last_test.go

broker/src/asapo_broker/server/get_next.go

broker/src/asapo_broker/server/get_next_test.go

broker/src/asapo_broker/server/get_size.go

broker/src/asapo_broker/server/get_size_test.go

broker/src/asapo_broker/server/listroutes.go

broker/src/asapo_broker/server/process_request.go

broker/src/asapo_broker/server/process_request_test.go

broker/src/asapo_broker/server/statistics_writers.go