Add missing locks in `net.{cpp,h}`:
* writing variable `nTotalBytesRecv` requires holding mutex `cs_totalBytesRecv` exclusively
* writing variables `nTotalBytesSent`, `nMaxOutboundTotalBytesSentInCycle` and `nMaxOutboundCycleStartTime` require holding mutex `cs_totalBytesSent` exclusively
* writing variables `nMaxOutboundTimeframe` and `nMaxOutboundLimit` require holding mutex `cs_totalBytesSent` exclusively
* writing variable `vAddedNodes` requires holding mutex `cs_vAddedNodes` exclusively