SMP Caching, part 2: Rock Store addition

Hello,

    The attached patch contains the second part of the SMP caching
changes described in the "SMP Rock Store and shared memory cache
patches" email. I labeled it [PREVIEW] because of the following
outstanding TODO item:

 * Portability tests and changes (via Hudson).

However, I do not expect the above item to cause significant changes to
the patch.

There are a few additional disk caching optimizations that we may
integrate soon or post later, after the current code is reviewed.

The patch preamble describes the overall Rock Store design. Please see
the following wiki page for more details:
http://wiki.squid-cache.org/Features/RockStore

For the complete set of changes (all three parts), and for testing,
please see the following Launchpad branch:
  lp:~measurement-factory/squid/3p2-rock

Thank you,

Alex.