Improve the subgraph performance using: immutable entities and using Bytes as the id of entities

[llunaCreixent]

here is now a new tool in our workbench to improve indexing speed, reduce data storage needs per subgraph & speed up some queries. Read more here: https://medium.com/edge-node-engineering/two-simple-subgraph-performance-improvements-a76c6b3e7eac