GC and TTLing blocks

Badger has support for TTL records.

We could use TTL on records, but we would also have to modify the blockstore so that any blocks read are written with a new TTL. This implies making sure that that only happens when blocks have been read for the gateway, and not for other things (i.e. providing them on bitswap, or announcing them to the dht).

Then badgerGC would automatically remove non-used blocks.

This doesn't ensure that you don't run out of space (i.e. lots of very recent blocks). It would be better if we could do "delete blocks older than X" rather than giving every block a TTL. In any case might need to be combined with other GC strategy.