Necessary for proper stdin functioning

Otherwise (especially with stdin) you sometimes end up in cases
where the high water mark is zero, and the current buffer is at 0,
and it doesn't need a readable event, so it never calls _read().

Otherwise resume() will cause data to be emitted before it can be handled.

This fixes the CONNECT/Upgrade HTTP functionality, which was not getting
sliced properly, because readable wasn't emitted on this tick.

Conflicts:

	test/simple/test-http-connect.js

Significant performance impact

It's too slow, unfortunately.

The Array.forEach call is too expensive.

Also, organize the numeric settings a bit on the ReadableState class