CLEANUP: conn_stream: rename cs_endpoint to sedesc (stream endpoint descriptor)

CLEANUP: conn_stream: rename cs_endpoint to sedesc (stream endpoint descriptor)

After some discussion we found that the cs_endpoint was precisely the
descriptor for a stream endpoint, hence the naturally coming name,
stream endpoint constructor.

This patch renames only the type everywhere and the new/init/free functions
to remain consistent with it. Future patches will address field names and
argument names in various code areas.