[core] Fixed MAVG msRcvBuf and msSndBuf

[maxsharabayko]

There was a mistake in the calculation of the moving average values of msRcvBuf and msSndBuf.

Note. Initial values of m_iCountMAvg, m_iBytesCountMAvg and m_TimespanMAvg are zero.
It means, at the start, the update will be:

m_iCountMAvg      = (int)((0 + (count * elapsed_ms)) / 1000);

Probably better to initialize with -1 and set the initial value to the instant value.
@mbakholdina What do you think?

Fixes #1272

[maxsharabayko]

Added handling of initial zero values.
Maybe makes sense to have m_iCountMAvg and m_iBytesCountMAvg of type uint64_t instead of int. @ethouris What do you think?

[maxsharabayko]

Maybe makes sense to have m_iCountMAvg and m_iBytesCountMAvg of type uint64_t instead of int.

Looks like int is ok-ish. m_iAckedBytesCount represents the current state of the buffer, not the accumulated count of all the bytes ever acknowledged.
In this aspect, int is more or less ok.