refactor: remove RecursiveMutex cs_totalBytesRecv, use std::atomic instead

The RecursiveMutex cs_totalBytesRecv is only used at two places: in
CConnman::RecordBytesRecv() to increment the nTotalBytesRecv member, and in
CConnman::GetTotalBytesRecv() to read it. For this simple use-case, we can
make the member std::atomic instead to achieve the same result.

src/net.h

@@ -1074,9 +1074,8 @@ private:
     static bool NodeFullyConnected(const CNode* pnode);
 
     // Network usage totals
-    mutable RecursiveMutex cs_totalBytesRecv;
     mutable RecursiveMutex cs_totalBytesSent;
-    uint64_t nTotalBytesRecv GUARDED_BY(cs_totalBytesRecv) {0};
+    std::atomic<uint64_t> nTotalBytesRecv{0};
     uint64_t nTotalBytesSent GUARDED_BY(cs_totalBytesSent) {0};
 
     // outbound limit & stats