change bandwidth type from int to int64 to prevent overflow

Closes #40185 (closed). Instead of using uint64 for the bandwidth unit (as I'd originally suggested), I used int64, because GetStat() in bytesNullLogger returns -1 for the inbound and outbound bandwidth values (otherwise we'd have to find a different way to indicate that bandwidth data wasn't logged, and int64 is already much more than enough for bandwidth logging).