Fix "amqp.exceptions.UnexpectedFrame: Received 0x69 while expecting 0xce" while using drain_events with timeout

Fix "amqp.exceptions.UnexpectedFrame: Received 0x69 while expecting 0xce" while using drain_events with timeout

This error is easy to reproduce on a slow connection to rabbitmq with many consumers and high message rate. If socket.timeout error occurs while reading payload:

    def read_frame(self, unpack=unpack):
        read = self._read
        try:
            frame_type, channel, size = unpack('>BHI', read(7, True))
            payload = read(size) # If socket.timeout occurs here 
            ch = ord(read(1))
        except socket.timeout:
            raise

part of payload copied to the self._read_buffer, and next "read_frame" call tries to read frame_header (frame_type, channel, size) from self._read_buffer which contains part of payload from previous read_frame call.

Possible code sample for error reproducing:
        while True:
            try:
                channel.connection.drain_events(timeout=0.1)
            except socket.timeout:
                # Do something else
            # Do something else

Possible exceptions:
    amqp.exceptions.UnexpectedFrame: Received 0x64 while expecting 0xce
    amqp.exceptions.UnexpectedFrame: Received frame 1 while expecting type: 3