Function edge_key 

pub fn edge_key(
    namespace: &str,
    from_id: &RecordId,
    edge_type: &str,
    to_id: &RecordId,
) -> Vec<u8> 

Build an edge storage key.

Format: [0x02][ns_hash: 4B][from_key_len: 2B][from_key][et_hash: 4B][to_key_len: 2B][to_key]