Pass existing Net_SFTP object into Net_SFTP_Stream.

Otherwise a new SFTP connection is created by the Stream Wrapper (with no public host key verification).

Follows #9718
Needs backport to stable7
stable6 is not affected (not using SFTP Stream Wrapper)
Ref https://github.com/phpseclib/phpseclib/blob/0.3.6/phpseclib/Net/SFTP/Stream.php#L173

@PVince81 @LukasReschke @icewind1991 @blizzz 
How global is `$GLOBALS` ?
Is it kept between PHP requests ?

Didn't we just want to restrict it to a single PHP request ? (maybe as static class attribute)
I thought the issue was that it was recreating the stream multiple times inside a single PHP request.