ChainIndexer: move lotus-shed chainindex into lotus CLI

[rvagg]

The new chain_index.go functionality in lotus-shed is more like a lotus command now than the previous sqlite db management utilities. It interacts via RPC and doesn't do any shonky multi-writer work anymore and our documentation treats it as a normal part of a workflow, so we should just normalise it.

Let's move it into the lotus CLI. I think it could either go in under the existing chain subcommand as an index sub-sub command, or it we could just add a new top-level index subcommand in the same DEVELOPER group as chain.

[rvagg]

Some thoughts on how to attack this:

• Start by looking at how the lotus CLI is instantiated - cmd/lotus/main.go - it pulls in its subcommands from cli/clicommands/cmd.go, which is where the initial groupings get set that you see in lotus help.
• I think we should just add a new subcommand, so it shows up in that DEVELOPER list as index Manage the chain indexer
• cmd/lotus-shed/chain_index.go has the existing command that we want to move, it's currently named chainindex but I think we should just be index for the new one. It has one sub-sub-command, validate-backfill, which is what we're moving. That file needs to move into the cli/ package, I think we should just call it index.go. Have a look at chain.go as a template for how it should be organised, and follow where ChainCmd is used, we want to add an IndexCmd.
• Again, look through chain.go for any patterns that we should be following once out of lotus-shed, hopefully they're roughly the same. We're connecting to the node via RPC but we need to figure out where to connect to, that's mostly all handled for us via GetFullNodeAPI and friends. Basically we want to make this command fit in to lotus, and hopefully it's pretty close as it is and this will mostly be about just moving the file and its wiring.