a (more) general cache facility that is safe(r) memory-wise

[Denys Duchier]

testing this cache facility for weave._extract without inventory caching on my
habitual example:

	time bzr log NEWS >/dev/null

I get quite good results with Cache(2) (2m46s) and somewhat less good results
with Cache(1) (3m17s).

Cheers,

--Denys