[BUG] LiveCC: Fix coefficients of smoothed average for AvgPayloadSize

[mbakholdina]

As per RFC, in LiveCC module the formula of calculating AvgPayloadSize should be the following

On sending a data packet (either original, or retransmitted), update
the value of average packet payload size AvgPayloadSize:

~~~
AvgPayloadSize = AvgPayloadSize * 0.875 + PacketPayloadSize * 0.125
~~~

where PacketPayloadSize is the payload size of a sent data packet, in bytes;
the initial value of AvgPayloadSize is equal the maximum allowed packet
payload size which can't be larger than 1456 bytes.

Currently, the coefficients are 127/128 and 1/128 what makes no sense when using smoothed average. Fix the coefficients to be 7/8 and 1/8.