Update to latest version of Guava
Tweak to cacheLoader to make a move to futures easier

The postCalc callback makes attaching onComplete events to a returned
future easier/more flexible - so we can add an onComplete with the
'fix' call.
record cache stats...
Futures for just the commits, not the trees?

Ideally though, we would like to parallise top-level trees,
because trees are much more susceptible to parallelisation - doing
every single tree entry with a future might lead to perf problems,
but top-level trees are currently par - and unlike commits, they don't
have the inherently sequentially-dependent nature of commits
blorg
treeblobs don't need to be cleaned with explicit concurrency...

The shallow-stack stuff doesn't apply to them, as Git repo folder depth is
rarely more than 100 (opposed to commit histories, which might be as deep
as 1000000 commits or more).

CPU cores will never be starved of work, as there are always other commits
and commit-trees to clean - no need to share cleaning of a signle tree.
Runs & passes tests... but benchmarks sloooow against wine
more stats collected
cut out the middleman, go direct

Perf verging on tolerable