gRPC callbacks are reentrant, defer them

[PiotrSikora]

gRPC callbacks are reentrant, leading to callback-during-callback, and crashes in SDKs.

Those should be deferred (similarly to callbacks for HTTP callouts, etc.)

Reported in proxy-wasm/proxy-wasm-rust-sdk#206, although plugins would crash even with deferred call due to unregistered token_id.

cc @mpwarres

[PiotrSikora]

@mpwarres could you take a stab at fixing this?