Add a new singleton ENTITY_COUNTS and add a post upgrade step to update this state

[Neeharika-Sompalli]

• Add a new singleton ENTITY_COUNTS state in EntityIdService that has a field (e.g. uint64 num_accounts or uint64 num_bytecodes) for each type of entity.
• Add a 0.59-only step in SystemSetup.doPostUpgradeSetup() that initializes all these counts using direct calls to KVState.size() for every K/V state used by every service.
• Update the WritableEntityIdStore API to "vend" entity id numbers by type, so that each time a service wants to create a new entity of a certain type, the EntityIdService can increment the correct ENTITY_COUNTS field.