server: Remove unneded node_id from IndexNodeServer

graphql: Add `subgraphError` argument only when feature is present

We saw an issue in a graphql client that is avoided by doing this.
and it is generally more correct to only include it if the subgraph opts in.
This requires storing the `features` field in the metadata, which we should be doing anyways.

node, store: Keep wait stats for each store/pool separately

node, store: Keep wait stats for each store/pool separately

all: Address review comments for load management

Comments are in Zac's review of
graphprotocol#1762