Issue #41 has shown a deadlock scenario where various object iterators
would wait for memory.

While reviewing this issue, we noticed a couple of shortcomings in the
existing code:

* fellow_cache_seg_ref_in() would always wait for allocation requests
  for readahead segments. Yet, when under memory pressure, we should
  not wait at all for memory for readahead.

* fellow_cache_obj_iter() would hold onto already sent segments also
  when waiting for synchronous I/O and memory allocations.

To improve on these shortcomings and further optimize the code, some
of fellow_cache_obj_iter() and all of the readahead code has been
rewritten. Improvements comprise the following:

* For read ahead, we now use asynchronous memory allocations. If they
  succeed right away, we issue I/O right away also, but if allocations
  are delayed, we continue delivery and check back later. By chance,
  memory allocations will succeed until then.

* We decouple memory allocations from specific segments and only care
  about the right size of the allocation. Because many segments will
  be of chunk_bytes size, this will allow more efficient use of
  available asynchronous allocations.

* We now de-reference already sent segments also whenever we need to
  wait for anything, be it a memory allocation or I/O. This should
  help overall efficiency and reduce memory pressure, because already
  sent segments can be LRUd earlier.

  The drawback is that we flush the VDP pipeline more often (we need
  to before we can deref segments).

We also cap the readahead parameter at the equivalent of 1/16 of
memory in order to avoid inefficiencies because of single requests
holding too much of the memory cache hostage.

An additional hard cap at 31 is required to keep the default esi depth
supported with the default stack size of varnish-cache.

we only set stobj->priv after returning from obj_get(), so
assert(oc->stobj->priv == fco) could trigger in the lru thread.

We now set the priv right before inserting into LRU.

It does not make sense to use up memory for busy objects if we can not
create their cache and disk counterparts in memory.

Motivated by #41

Motivated by https://gitlab.com/uplex/varnish/slash/-/issues/41#note_1688912442

Also added to Varnish-Cache: https://github.com/varnishcache/varnish-cache/commit/24b434383c616639d5aa9be9b5ba3647a418d64c

Fixes #42

to enable parallel tests, for example:

$ for i in {1..24} ; do while ./src/fellow_cache_test /tmp/f.${i} >/dev/null 2>/dev/null ; do : ;done & done ; wait

Motivated by #42

Have seen (fbo) != NULL in fellow_cache_async_write_complete():

 #14 0x00007f9aa30957e5 in fellow_cache_async_write_complete (fc=0x7f9aa2c41300, ptr=0x7f9a9ff4df58, result=4096)
    at fellow_cache.c:2791
 #15 0x00007f9aa3096403 in fellow_cache_seg_async_compl_cb (priv=0x7f9aa2c41300, status=0x7f9a999fa3e0, n=1)
    at fellow_cache.c:2951

(gdb) info local
fbio = 0x7f9a9ff4df58
fbo = 0x0
fcs = 0x0
fco = 0x7f9a9ff4f000
fcos_next = FCOS_INVAL
type = FBIO_SEG
io_outstanding = 2 '\002'
refcount = 0
__PRETTY_FUNCTION__ = "fellow_cache_async_write_complete"
lcb = {{magic = 2863944409, n_add = 0, l_rem = 2, n_rem = 0, fco = 0x7f9a9ff4f000, add = {vtqh_first = 0x0,
      vtqh_last = 0x7f9a999fa2b8}, fcs = 0x7f9a9ff4df5c}}
__func__ = "fellow_cache_async_write_complete"
_pterr281611 = <optimized out>
_pterr282913 = <optimized out>
_pterr289715 = <optimized out>
(gdb) p *fbio
$1 = {magic = 3019, retries = 0, type = FBIO_SEG, sync = FBIOS_ASYNC, fbo = 0x0, u = {fcs = 0x0, seglist = {fdsl = 0x0,
      reg = {off = 0, size = 0}}}}

we can have FCS_BUSY segments before OBJ_ITER_END while streaming.

At the last segment, do not advance to the next segment list if it is
still empty.

we only jump to the again label if we did not get a reference.

Motivated by #40

CID#469253

CID#469261

Coverity CID#469242

Coverity CID#469229

To address some Coverity pedentry with minor impact (at most)

CID#469230

Coverity CID#469233

Spotted by Coverity CID#469228, but it is irrelevant because only in
test code

Good catch by Coverity CID#469254

Coverity CID#469252

Ref Coverity CID#469262

The optimized case for multiple segments from the same fco did
not work as expected, continue did not continue the inner loop.

Spotted by Coverity, CID#469236

Coverity CID#469225

Ref CID#469268

Motivated by #41