Add a "compact" command

[arajasek]

Inspired by #4547, our docs currently say:

## Compacting the chain data

It is possible to "prune" the current chain data used by Lotus to reduce the disk footprint of a Lotus node by resyncing from a minimal snapshot. This is a quick overview of the steps:

1. Stop the lotus daemon, i.e. `lotus daemon stop`)
2. Remove the contents of the `datastore/chain/` folder in the Lotus path, i.e. `rm -rf ~/.lotus/datastore/chain/*`.
3. Start the daemon using a minimal snapshot as explained above, i.e. `lotus daemon --import-snapshot https://fil-chain-snapshots-fallback.s3.amazonaws.com/mainnet/minimal_finality_stateroots_latest.car`.

We probably shouldn't be telling people to directly delete folders. Adding some lotus compact command that does this in one step is probably the best thing to do.

[jennijuju]

Another solution is actually have an option to trigger regular chain datastore pruning

[arajasek]

This should largely get implemented by #4992

[jennijuju]

Add documentation for split blockstore, which implements this feature by nature.

[vyzo]

Note that as part of #5808 I am adding a lotus-shed command to gc the hotstore.
We can do this as an additional command in that vein, that garbage collects the coldstore.
We will have to finish #5808 first however, it still has a couple of small issues with hotstore misses.

[parkan]

running the daemon with --import-snapshot a second time appears to blow away the chain data and restart the sync, would that be equivalent to the steps in the docs, or is it more destructive?

I'll open another issue to warn on unintentional --import-snapshot if not

(brought to you by restarting the daemon to reload config and unintentionally restarting the sync)

[arajasek]

Splitstore is the correct way to do this, we can close this.