DEBUG: stream: minor rearrangement of a few fields in struct stream.

DEBUG: stream: minor rearrangement of a few fields in struct stream.

Some recent traces started to show confusing stream pointers ending with
0xe. The reason was that the stream's obj_type was almost unused in the
past and was stuffed in a hole in the structure. But now it's present in
all "show sess all" outputs and having to mentally match this value against
another one that's 0x17e lower is painful. The solution here is to move the
obj_type at the top, like in almost every other structure, but without
breaking the efficient layout.

This patch moves a few fields around and manages to both plug some holes
(16 bytes saved, 976 to 960) and avoid channels needlessly crossing cache
boundaries (res was spread over 3 lines vs 2 now).

Nothing else was changed. It would be desirable to backport this to 2.6
since it's where dumps are currently being processed the most.