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;
 }