stream: remove lowWaterMark feature

It seems like a good idea on the face of it, but lowWaterMarks are
actually not useful, and in practice should always be set to zero.

It would be worthwhile for writers if we actually did some kind of
writev() type of thing, but actually this just delays calling write()
and the overhead of doing a bunch of Buffer copies is not worth the
slight benefit of calling write() fewer times.
stream: Return false from push() more properly

There are cases where a push() call would return true, even though
the thing being pushed was in fact way way larger than the high
water mark, simply because the 'needReadable' was already set, and
would not get unset until nextTick.

In some cases, this could lead to an infinite loop of pushing data
into the buffer, never getting to the 'readable' event which would
unset the needReadable flag.

Fix by splitting up the emitReadable function, so that it always
sets the flag on this tick, even if it defers until nextTick to
actually emit the event.

Also, if we're not ending or already in the process of reading, it
now calls read(0) if we're below the high water mark.  Thus, the
highWaterMark value is the intended amount to buffer up to, and it
is smarter about hitting the target.