Allow to disable optimistic send in PushMessage()

Profiling has shown that optimistic send causes measurable slowdowns when
many messages are pushed, even if the sockets are non-blocking. Better to
allow disabling of optimistic sending in such cases and let the network
thread do the actual socket calls.

src/net.cpp

@@ -3054,7 +3054,7 @@ bool CConnman::NodeFullyConnected(const CNode* pnode)
     return pnode && pnode->fSuccessfullyConnected && !pnode->fDisconnect;
 }
 
-void CConnman::PushMessage(CNode* pnode, CSerializedNetMsg&& msg)
+void CConnman::PushMessage(CNode* pnode, CSerializedNetMsg&& msg, bool allowOptimisticSend)
 {
     size_t nMessageSize = msg.data.size();
     size_t nTotalSize = nMessageSize + CMessageHeader::HEADER_SIZE;
@@ -3071,7 +3071,7 @@ void CConnman::PushMessage(CNode* pnode, CSerializedNetMsg&& msg)
     size_t nBytesSent = 0;
     {
         LOCK(pnode->cs_vSend);
-        bool optimisticSend(pnode->vSendMsg.empty());
+        bool optimisticSend(allowOptimisticSend && pnode->vSendMsg.empty());
 
         //log total amount of bytes per command
         pnode->mapSendBytesPerMsgCmd[msg.command] += nTotalSize;

src/net.h

@@ -201,7 +201,7 @@ class CConnman
 
     bool IsMasternodeOrDisconnectRequested(const CService& addr);
 
-    void PushMessage(CNode* pnode, CSerializedNetMsg&& msg);
+    void PushMessage(CNode* pnode, CSerializedNetMsg&& msg, bool allowOptimisticSend = true);
 
     template<typename Condition, typename Callable>
     bool ForEachNodeContinueIf(const Condition& cond, Callable&& func)