Update indexer to replace the deprecated the rpc endpoint `/chains/<chain_id>/blocks/<block_id>/context/delegates/<baker_pkh/staking_balance`

As per https://tezos.gitlab.io/protocols/021_quebec.html#rpc-changes, the /chains/<chain_id>/blocks/<block_id>/context/delegates/<baker_pkh/staking_balance endpoint is deprecated. The suggested update is to

call RPCs GET ../total_staked and GET ../total_delegated, and add their outputs together.

So we should replace the call to this endpoint in the indexer with these two calls. May be we can do this without changing the node interface by managing this in low level.