chore(blockstore): add an LRU cache to the API blockstore wrapper

[rvagg]

I've been relying on NewAPIBlockstore a lot lately for lotus-shed commands, miscellaneous things to inspect state mainly, where I don't want to stop my node to mount it as the blockstore. But realised that a lot of the fetches are for the same small blocks because we have a lot of things like THE empty partition, THE empty deadline, THE empty HAMT. It turns out that putting a simple LRU in there makes it so much faster to do simple iteration and checking when you're hitting these things.

This is mainly used in lotus-shed, but it has a few uses in the lotus command, specifically for some things under lotus filplus, lotus miner info and lotus multisig. The spcli commands also use this a little.

[masih]

No blockers.