streams: Support objects other than Buffers

We detect for non-string and non-buffer values in onread and
turn the stream into an "objectMode" stream.

If we are in "objectMode" mode then howMuchToRead will
always return 1, state.length will always have 1 appended
to it when there is a new item and fromList always takes
the first value from the list.

This means that for object streams, the n in read(n) is
ignored and read() will always return a single value

Fixed a bug with unpipe where the pipe would break because
the flowing state was not reset to false.

Fixed a bug with sync cb(null, null) in _read which would
forget to end the readable stream
streams2: Handle sync read callbacks nicely
test: Add test for negative stream drain counter