Add option, mostly for comparison's sake, that allows a deploy orchestrated by CombinedBlockCache such that its L1 is LruBlockCache for META blocks but DATA blocks are fetched each time (we don't try and cache them, no blockcache churn).
In operation, i can see fscache coming around to cover the fetched DATA blocks such that if the DATA blocks fit in fscache, seeks go to zero.
This setup for sure runs slower. Will publish numbers elsewhere. Meantime, here is a patch to enable this option.