* src/client: Make subscribe stream capacity explicit

Makes footgun on `Client::subscribe` explicit.

> Note that once the capacity of the returned [`Stream`] is reached, the
> background task blocks and thus all work related to the [`Client`] will
> pause until elements from the [`Stream`] are consumed and thus capacity
> is freed. Callers of [`Client::subscribe`] should either set a high
> capacity, continuously read from the returned [`Stream`] or drop it.