don't rely on drand networks for past beacons

[LesnyRumcajs]

Summary

An excellent summary is provided in filecoin-project/lotus#12527 (comment)

tl;dr we shouldn't rely on drand providing endpoints for historical beacons. It's both not scalable, incorrect in certain epochs and not reliable.

The downside of this is reduced performance when requesting beacon entries for really old epochs, e.g., first epoch of calibnet.

Completion Criteria

• Identify places where we call the drand servers for beacon data - if the data is available on chain, we should not do this and get that tipset's drand entry. There is at least one -

  forest/src/rpc/methods/state.rs

  Lines 1725 to 1758 in 5fd1ec5

  	impl RpcMethod<1> for StateGetBeaconEntry {
  	const NAME: &'static str = "Filecoin.StateGetBeaconEntry";
  	const PARAM_NAMES: [&'static str; 1] = ["epoch"];
  	const API_PATHS: ApiPaths = ApiPaths::V1;
  	const PERMISSION: Permission = Permission::Read;
  	type Params = (ChainEpoch,);
  	type Ok = BeaconEntry;
  	async fn handle(
  	ctx: Ctx<impl Blockstore>,
  	(epoch,): Self::Params,
  	) -> Result<Self::Ok, ServerError> {
  	{
  	let genesis_timestamp = ctx.chain_store().genesis_block_header().timestamp as i64;
  	let block_delay = ctx.chain_config().block_delay_secs as i64;
  	// Give it a 1s clock drift buffer
  	let epoch_timestamp = genesis_timestamp + block_delay * epoch + 1;
  	let now_timestamp = chrono::Utc::now().timestamp();
  	match epoch_timestamp.saturating_sub(now_timestamp) {
  	diff if diff > 0 => {
  	tokio::time::sleep(Duration::from_secs(diff as u64)).await;
  	}
  	_ => {}
  	};
  	}
  	let (_, beacon) = ctx.beacon().beacon_for_epoch(epoch)?;
  	let network_version = ctx.state_manager.get_network_version(epoch);
  	let round = beacon.max_beacon_round_for_epoch(network_version, epoch);
  	let entry = beacon.entry(round).await?;
  	Ok(entry)
  	}
  	}

• assert that performance is similar to Lotus's, both calibnet and mainnet.

Additional Links & Resources

[LesnyRumcajs]

Also, on mainnet, getting old entry doesn't work.

❯ time curl --silent -X POST -H "Content-Type: application/json" \
             --data '{"jsonrpc":"2.0","id":2,"method":"Filecoin.StateGetBeaconEntry","params": [ 1 ] }' \
             "http://127.0.0.1:2345/rpc/v1"
{"jsonrpc":"2.0","id":2,"error":{"code":-32603,"message":"Aggregated errors:\n"}}
________________________________________________________
Executed in  884.48 secs      fish           external
   usr time    8.36 millis  384.00 micros    7.98 millis
   sys time   21.31 millis   92.00 micros   21.21 millis