net: fix a few races. Credit @TheBlueMatt

These are (afaik) all long-standing races or concurrent accesses. Going
forward, we can clean these up so that they're not all individual atomic
accesses.

- Reintroduce cs_vRecv to guard receive-specific vars
- Lock vRecv/vSend for CNodeStats
- Make some vars atomic.
- Only set the connection time in CNode's constructor so that it doesn't change

src/net_processing.cpp

@@ -2450,7 +2450,7 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
                     if (pingUsecTime > 0) {
                         // Successful ping time measurement, replace previous
                         pfrom->nPingUsecTime = pingUsecTime;
-                        pfrom->nMinPingUsecTime = std::min(pfrom->nMinPingUsecTime, pingUsecTime);
+                        pfrom->nMinPingUsecTime = std::min(pfrom->nMinPingUsecTime.load(), pingUsecTime);
                     } else {
                         // This should never happen
                         sProblem = "Timing mishap";