fix(client): `disconnect_reason/read_error` is cancel-safe (#1347)

* client: `disconnect_reason/read_error` cancel-safe

If/when the connection is closed, the cause is fetched by `read_error` from the background task.
It was not cancel-safe, which could have side-effects and mutate the state,
such as the internal Option could be `None` and cause a panic.

* fix wasm build

* remove async_lock dependency