stream: Don't require read(0) to emit 'readable' event

When a readable listener is added, call read(0) so that data will flow in, up to
the high water mark.

Otherwise, it's somewhat confusing that you have to listen for readable,
and ALSO call read() (when it will certainly return null) just to get some
data out of the stream.

See: #4720

/cc @TooTallNate