Previous code would only deliver fully received segments even if a
busy object was being written to by the backend side (streaming).

I guess at some point before the public release I must have thought
about this and decided that streaming only completed segments should
be acceptable at least to begin with, but I overlooked the fact that
the previous implementation could lead to short body writes due to a
lack of coordination between the writing and reading side.

This commit introduces proper streaming support, also within busy
segments.

In particular, this should also solve an issue reported by @tomazz75
on gitlab where no body was sent at all. This problem could lead to
the client stalling on a wait for body data which never came.

The nature of this problem was a race condition, which I was only able
to reproduce on my system with the following patch to favor the race:

diff --git a/src/fellow_cache.c b/src/fellow_cache.c
index 3073b35..c9d9a4e 100644
--- a/src/fellow_cache.c
+++ b/src/fellow_cache.c
@@ -3338,6 +3338,9 @@ fellow_busy_obj_getspace(struct fellow_busy *fbo, size_t *sz, uint8_t **ptr)
        assert(*sz > 0);
        AN(ptr);

+       // XXX DEBUG
+       usleep(10000);
+
        CHECK_OBJ_NOTNULL(fbo->fc, FELLOW_CACHE_MAGIC);
        CHECK_OBJ_NOTNULL(fbo->fc->tune, STVFE_TUNE_MAGIC);
        max = (size_t)1 << fbo->fc->tune->chunk_exponent;

This fix survived 1000 calls to the respective test case:

	/tmp/bin/varnishtest \
		-Dlibvmod_slash=/tmp/lib/varnish/vmods/libvmod_slash.so \
		src/vtc/fellow_c00093.vtc \
		-n 1000 -j 20

Thank you to @tomazz75 for reporting the bug and help with additional
information to track it down.

Fixes #2