Loading splitstore as universal blockstore not working properly in lotus-shed

[rvagg]

In #12703 I had to modify loadChainStore in cmd/lotus-shed/state-stats.go to make it work with my splitstore node. Prior to "fixing" it there, it would start up, load the latest tipset from the metadata store, and then fail to load the tipset from the blockstore (not found). I added that change and got it working. In the conversation in that PR, #12703 (comment), @ZenGround0 pointed out that the existing code should have worked, and in fact, this exact code path used to work in 2023. So something's sus and needs to be investigated.

1. Investigate this issue and work out why the universal blockstore access isn't working for a splitstore node
2. Do some unification of the loading logic in lotus-shed - this same pattern is repeated many times across the code, there's both forms