gRPC callbacks are reentrant, defer them #373
Description
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