mempool tidyup commit

I'm now committing Andres' mempool tidyups. I have tagged the repo
with the tag PRE_ANDRE_MEMPOOL_TIDYUP_1 before I commit, so we can
rollback if there are any serious problems.

People are very much encouraged to give me a hand here in testing
the usefulness of these patches in squid's memory 'leakage'.

Thanks!

-- 
Adrian Chadd			"It was then that I knew that I wouldn't
<adrian@creative.net.au>	    die, as a doctor wouldn't fart in front
				      of a dying boy." -- Angela's Ashes