BufferedTee provides reader interface to the consumer. Data read by consumer
is also written to the output. Output is hold lookahead_size bytes behind
consumer. Allowing consumer to put back some bytes to be read again. On flush
all consumed bytes are flushed to the output.

      input   ->   tee   ->   consumer
                    |
                 output

input - underlying unbuffered reader
output - writer, receives data read by consumer
consumer - uses provided reader interface

If lookahead_size is zero output always has same bytes as consumer.