[RPC-Spec-V2] Limit `chainHead_unstable_follow` subscriptions

[lexnv]

The JSON-RPC server must accept at least 2 chainHead_unstable_follow subscriptions per JSON-RPC client. Trying to open more might lead to a JSON-RPC error when calling chainHead_unstable_follow. In other words, as long as a JSON-RPC client starts 2 or fewer chainHead_unstable_follow subscriptions, it is guaranteed that this return value will never happen.

• ensure that at least 2 subscriptions per RPC client are accepted by the server

// @paritytech/subxt-team