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.cpp

@@ -2879,7 +2879,6 @@ bool CConnman::DisconnectNode(NodeId id)
 
 void CConnman::RecordBytesRecv(uint64_t bytes)
 {
-    LOCK(cs_totalBytesRecv);
     nTotalBytesRecv += bytes;
 }
 
@@ -2956,7 +2955,6 @@ uint64_t CConnman::GetOutboundTargetBytesLeft() const
 
 uint64_t CConnman::GetTotalBytesRecv() const
 {
-    LOCK(cs_totalBytesRecv);
     return nTotalBytesRecv;
 }
 

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