Clients to the Object API need to know not only the current extension
(new length) of streaming objects, but also the streaming state - in
particular BOS_FINISHED and BOS_FAILED. The latter for obvious
reasons, and the former to call the delivery function with
OBJ_ITER_END, which then likely results in VDP_END sent down the
delivery pipeline.

Background:

It is important for efficient delivery to not receive an additional
VDP_END with a null buffer, but rather combined with the last chunk of
data, so, consequently, it is important to reliably send OBJ_INTER_END
also with the last chunk of data.

Consequent to all of this, ObjWaitExtend() callers need to know when
BOS_FINISHED has been reached for some extension.

The current API, however, does not provide a consistent view of the
streaming state, which is only available from within the critical region
of ObjWaitExtend().

Thus, we add the streaming state as an optional return value.

With this commit, we also remove a superfluous line to set rv again:
Because boc->fetched_so_far must only be updated while holding the boc
mutex, reading the value again provides no benefit.