Properly handle large reads from push-streams

Problem 1: If stream.push() triggers a 'readable' event, and the user
calls `read(n)` with some n > the highWaterMark, then the push() will
return false (indicating that they should not push any more), but no
future 'readable' event is coming (because we're above the
highWaterMark).

Solution: return true from push() when needReadable is set.

Problem 2: A read(n) for n != 0, after the stream had encountered an
EOF, would not trigger the 'end' event if the EOF was pushed in
synchronously by the _read() function.

Solution: Check for ended in stream.read() and schedule an end event if
the length now equals 0.

Fix #4585