client: ignore rc_buffer clippy warning

[Fishrock123]

This new lint popped up overnight, see https://github.com/rust-lang/rust-clippy/pull/6044

The problem is that we do indeed want this structure.

• The outer Arc allows us to clone in .send() without cloning the array.
• The Vec allows us to add middleware at runtime.
• The inner Arc-s allow us to implement Clone without sharing the vector with the parent.
• We don't use a Mutex around the Vec here because adding a middleware during execution should be an error.