Fix missing flush in StreamCopyThread

It is possible that StreamCopyThread will not flush everything
from it's src to it's dst.  In most cases StreamCopyThread works
like this:

  in loop:
    n = src.read(buf);
    dst.write(buf, 0, n);

and when we want to flush, we interrupt() StreamCopyThread and it
flushes everything it wrote to dst.

The problem is that our interrupt() could interrupt reading. In this
case we will flush everything we wrote to dst, but not everything
we wrote to src.

Change-Id: Ifaf4d8be87535c7364dd59b217dfc631460018ff
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>