This PR attempts to provide the functionality requested in #2288. I opted to follow the code used i nix-collect-garbage but factored it into a private method on LocalStorage to avoid cluttering optimiseStore.

I do have one question that might impact this PR beyond any organizational or stylistic blunders: How accurately is accurate enough for on disk storage? nix-collect-garbage uses st_blocks * 512ULL, which will be accurate for the usual case. optimiseStore uses st_size, which will be similar (though possibly less honest about the disk usage).

In a case with a block size that isn't the usual 512 (i.e. networked storages, etc.), the reported number could not reflect the actual size on disk well at all, which would beg for a query to st_blksize

For now, I used st_size to be consistent within optimise-store.cc, which means it gives a slightly different answer than the same text in used in gc.cc.

Happy to make any changes and correct rookie mistakes.