Use buffer.rewind() instead of buffer.clear() in Piece._read()

Buffer.clear() was a confusing call as it felt like the data read into
the buffer was discarded (although it's not). Buffer.rewind() is a much
better use of the Buffer API.

Suggested-by: Carl Ericsson <carl.ericsson@gmail.com>
Signed-off-by: Maxime Petazzoni <mpetazzoni@turn.com>