This commit mostly just moves some code and modules around for better organization, but there are some functional changes.
stream manager related code now lives in the 'stream' module.
stream module exports public symboles using all and init.py Users should import stream manager symbols like:
from eventutilities_python.stream import stream_manager, load_config, etc.
Stream.close has been removed from the interface. Closing is handled by stream_manager.
flink.py should contain only contain Flink specific things. As such, EventStreamDescriptor has been removed (importing it would cause a circular import). This lower level APIs use stream_name and schema_version, similar to the Java interface. If we want to make EventStreamDescriptor a top level Java interface, we can. Or use and wrap java EventStream class instead.