stream: Start out in sync=true state

The Readable and Writable classes will nextTick certain things
if in sync mode.  The sync flag gets unset after a call to _read
or _write.  However, most of these behaviors should also be
deferred until nextTick if no reads have been made (for example,
the automatic '_read up to hwm' behavior on Readable.push(chunk))

Set the sync flag to true in the constructor, so that it will not
trigger an immediate 'readable' event, call to _read, before the
user has had a chance to set a _read method implementation.