[PATCH] Fixes to get more collapsed forwarding hits

Hello,

    The attached patch contains several changes to improve the
probability of getting a collapsed forwarding hit:

* Allow STORE_MEMORY_CLIENTs to open disk files if needed and possible.
STORE_*_CLIENT designation is rather buggy (several known XXXs). Some
collapsed clients are marked as STORE_MEMORY_CLIENTs (for the lack of
info at determination time) but their hit content may actually come from
a disk cache.

* Do not abandon writing a collapsed cache entry when we cannot cache
the entry in RAM if the entry can be cached on disk instead. Both shared
memory cache and the disk cache have to refuse to cache the entry for it
to become non-collapsible. This dual refusal is difficult to detect
because each cache may make the caching decision at different times.
Added StoreEntry methods to track those decisions and react to them.

* Recognize disk cache as a potential source of the collapsed entry when
the memory cache is configured. While collapsed entries would normally
be found in the shared memory cache, caching policies and other factors
may prohibit memory caching but still allow disk caching. Memory cache
is still preferred.

* Do not use unknown entry size in StoreEntry::checkTooSmall()
determination. The size of collapsed entries is often unknown, even when
they are STORE_OK (because swap_hdr_sz is unknown when the other worker
has created the cache entry). The same code has been using this
ignore-unknowns logic for the Content-Length header value, so the
rejection of unknown entry size (added as a part of C++ conversion
without a dedicated message in r5766) could have been a typo.

If approved, I will commit the change for the last bullet separately
because it is easier to isolate and is less specific to collapsed
forwarding.

The patch does not attempt to cover all possible collapsing cases that
Squid currently misses, of course. More work remains in this area
(including a serious STORE_*_CLIENT redesign), but the proposed fixes
should make a dent.

These changes were developed for an earlier trunk version but the
trunk-ported patch posted here also passed simple lab tests.

Thank you,

Alex.