wip: streaming interface python <-> java

java/server/pom.xml

.../main/java/org/softwareheritage/graph/Py4JEntryPoint.java

.../java/org/softwareheritage/graph/algo/NodeIdConsumer.java

...java/org/softwareheritage/graph/algo/NodeIdsConsumer.java

...in/java/org/softwareheritage/graph/algo/PathConsumer.java

...java/org/softwareheritage/graph/algo/StreamTraversal.java

.../main/java/org/softwareheritage/graph/algo/Traversal.java

.../src/main/java/org/softwareheritage/graph/backend/Pp.java

...erver/src/main/java/org/softwareheritage/graph/py4jcli.py

...src/main/java/org/softwareheritage/graph/t/WriteLong.java

...rc/main/java/org/softwareheritage/graph/t/aiohttp_rest.py

...erver/src/main/java/org/softwareheritage/graph/t/data.bin

...r/src/main/java/org/softwareheritage/graph/t/read_long.py