The old implementation calls at least 2 read(2) (4bytes length and
payload) to receive the message. This change will read as many bytes
as possible in one read(2) call. BtPieceMessage::data_ is now just a
const pointer to the internal buffer of PeerConnection.
If the number of pieces gets bigger, the length of Bitfield message
payload exceeds the initial buffer capacity of PeerConnection, which
is MAX_PAYLOAD_LEN. We expand buffer as necessary so that
PeerConnection can receive the Bitfield message.