[RPC-Spec-V2] Limit ongoing operations

[lexnv]

The chainHead must guarantee that at least 16 concurrent operations can be processed:

The JSON-RPC server must accept at least 16 concurrent operations for any given [chainHead_unstable_follow] subscription. In other words, as long as the JSON-RPC client makes sure that no more than 16 operations are in progress at any given item, it is guaranteed that all of its operations will be accepted by the JSON-RPC server.

For this purpose, each item requested through [chainHead_unstable_storage] counts as one operation, and each call to [chainHead_unstable_body] and [chainHead_unstable_call] counts as one operation.

• ensure that at least 16 operations can be added to the internal queue
• provide back to the methods a limitReached result in case the RPC server cannot perform more ops
• discard any number of storage items that exceed the internal limit via discardedItems field

// @paritytech/subxt-team