rpc v2: backpressure chainHead_v1_storage

[niklasad1]

Close https://github.com/paritytech/polkadot-sdk/issues/5589

This PR makes it possible for rpc_v2::Storage::query_iter_paginated to be "backpressured" which is achieved by having a channel where the result is sent back and when this channel is "full" we pause the iteration.

The chainHead_follow has an internal channel which doesn't represent the actual connection and that is set to a very small number (16). Recall that the JSON-RPC server has a dedicate buffer for each connection by default of 64.

Notes

• Because archive_storage also depends on rpc_v2::Storage::query_iter_paginated I had to tweak the method to support limits as well. The reason is that archive_storage won't get backpressured properly because it's not an subscription. (it would much easier if it would be a subscription in rpc v2 spec because nothing against querying huge amount storage keys)
  • query_iter_paginated doesn't necessarily return the storage "in order" such as
• query_iter_paginated(vec![("key1", hash), ("key2", value)], ...) could return them in arbitrary order because it's wrapped in FuturesUnordered but I could change that if we want to process it inorder (it's slower)
• there is technically no limit on the number of storage queries in each chainHead_v1_storage call rather than the rpc max message limit which 10MB and only allowed to max 16 calls chainHead_v1_x concurrently (this should be fine)

Benchmarks using subxt on localhost

• Iterate over 10 accounts on westend-dev -> ~2-3x faster
• Fetch 1024 storage values (i.e, not descedant values) -> ~50x faster
• Fetch 1024 descendant values -> ~500x faster

The reason for this is because as Josep explained in the issue is that one is only allowed query five storage items per call and clients has make lots of calls to drive it forward..

[paritytech-cicd-pr]

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