Bug 7: Update cached entries on 304 responses.

Bug 7: Update cached entries on 304 responses.

New Store API to update entry metadata and headers on 304s.
Support entry updates in shared memory cache and rock cache_dirs.
No changes to ufs-based cache_dirs: Their entries are still not updated.

* Atomic StoreEntry metadata updating

   StoreEntry metadata (swap_file_sz, timestamps, etc.) is used
   throughout Squid code. Metadata cannot be updated atomically because
   it has many fields, but a partial update to those fields causes
   assertions. Still, we must update metadata when updating HTTP
   headers. Locking the entire entry for a rewrite does not work well
   because concurrent requests will attempt to download a new entry
   copy, defeating the very HTTP 304 optimization we want to support.

   Ipc::StoreMap index now uses an extra level of indirection (the
   StoreMap::fileNos index) which allows StoreMap control which
   anchor/fileno is associated with a given StoreEntry key. The entry
   updating code creates a disassociated (i.e., entry/key-less) anchor,
   writes new metadata and headers using that new anchor, and then
   _atomically_ switches the map to use that new anchor. This allows old
   readers to continue reading using the stale anchor/fileno as if
   nothing happened while a new reader gets the new anchor/fileno.

   Shared memory usage increase: 8 additional bytes per cache entry: 4
   for the extra level of indirection (StoreMapFileNos) plus 4 for
   splicing fresh chain prefix with the stale chain suffix
   (StoreMapAnchor::splicingPoint). However, if the updated headers are
   larger than the stale ones, Squid will allocate shared memory pages
   to accommodate for the increase, leading to shared memory
   fragmentation/waste for small increases.

* Revamped rock index rebuild process

   The index rebuild process had to be completely revamped because
   splicing fresh and stale entry slot chain segments implies tolerating
   multiple entry versions in a single chain and the old code was based
   on the assumption that different slot versions are incompatible. We
   were also uncomfortable with the old cavalier approach to accessing
   two differently indexed layers of information (entry vs. slot) using
   the same set of class fields, making it trivial to accidentally
   access entry data while using slot index.

   During the rewrite of the index rebuilding code, we also discovered a
   way to significantly reduce RAM usage for the index build map (a
   temporary object that is allocated in the beginning and freed at the
   end of the index build process). The savings depend on the cache
   size: A small cache saves about 30% (17 vs 24 bytes per entry/slot)
   while a 1TB cache_dir with 32KB slots (which implies uneven
   entry/slot indexes) saves more than 50% (~370MB vs. ~800MB).

   Adjusted how invalid slots are counted. The code was sometimes
   counting invalid entries and sometimes invalid entry slots. We should
   always count _slots_ now because progress is measured in the number
   of slots scanned, not entries loaded. This accounting change may
   surprise users with much higher "Invalid entries" count in cache.log
   upon startup, but at least the new reports are meaningful.

   This rewrite does not attempt to solve all rock index build problems.
   For example, the code still assumes that StoreEntry metadata fits a
   single slot which is not always true for very small slots.