Aggregate child checkpoints in a single CID

[adlrocha]

In the current implementation, we include inside ChildChecks a list of the CIDs for children for a specific checkpoint period. If the number of children is large, or if any of them uses a high-throughput consensus algorithm (like Mir) we may end up with a large number of CIDs in this list.

eudico/chain/consensus/hierarchical/checkpoints/schema/checkpoint.go

Lines 67 to 77 in 137c6dd

	type ChildCheck struct {
	Source string
	// NOTE: Same problem as below, checks is
	// []cid.Cid, but we are hiding it behind a bunch
	// of bytes to prevent the VM from trying to fetch the
	// cid from the state tree. We still want to use IPLD
	// for now. We could fix this by setting an empty AMT
	// with the list of Cids, but it may be too complex just
	// for the sake of using CBOR.
	Checks [][]byte //[]cid.Cid
	}

We should aggregate the number of child checkpoints in a single CID to prevent a state explosion. For this, we can either use a single CID aggregating all ChildChecks; we can include this data into the CrossMsgs information and propagate it inside the CrossMsgMeta; or keep a list of ChildChecks with a single CID for each source.

In the process we need to also switch to cbor-gen for serialization instead of IPLD schema, and fix the interoperability of checkpoint signatures