Add TCP send buffer and receive buffer usage to connection stats

Summary

We can't easily tell whether the TCP send and receive buffers are full which can delay when the DCP step function is run. Therefore it is useful to have the stats available to the connection stats (and when the connections are closed).

Detail

The stats are available using ioctl passing in the ioctl_type of SIOCOUTQ and SIOCINQ
See https://linux.die.net/man/7/tcp for further information.