Fix races for streaming busy objects

For streaming busy objects, we basically rely on the varnish-cache
ObjExtend() / ObjWaitExtend() API to never read past the object: In
fellow_stream_f(), we always wait for more data (or the end of the
object) before returning, such that fellow_cache_obj_iter(), which
iterates over segments, should never touch a segment past the final
FCS_BUSY segment.

Yet - it did, by means of the read-ahead and the peek-ahead to determine
whether or not OBJ_ITER_END should be signaled.

We fix this issue by reading/peeking ahead only for segments with a
state beyond FCS_BUSY.

There is now also extensive test infrastructure to specifically test
concurrent access ti busy objects. To keep layers separate,
fellow_cache_test uses a lightweight signal/wait implementation
analogous to the ObjExtend() / ObjWaitExtend() Varnish-Cache
interface.

An earlier version of t_busyobj() had run on my dev laptop for 3.5
hours without crashing, while without the fixes it had run into
assertion failures within seconds.

Fixes #35 and #36 (I hope)