Slow handling of connection incoming buffer

A connection incoming buffer is extended using += operator. This leads
to unnecessary memory allocations and significant performance drops when
transferring big amounts of data (i. e. a few megabytes). Using of the array('c')
(an array of bytes) data type enables us to handle incoming data efficiently.

I used this dummy code for testing:
    worker:  http://xsnippet.org/359371/
    client: http://xsnippet.org/359372/
    binary data: dd if=/dev/urandom of=data.bin bs=512 count=10000