A nsIInputStreamTee is a wrapper for an input stream, that when read
reads the specified amount of data from its |source| and copies that
data to its |sink|. |sink| must be a blocking output stream.
If |eventTarget| is set, copying to sink is done asynchronously using
the event-target (e.g. a thread). If |eventTarget| is not set, copying
to sink happens synchronously while reading from the source.