Use std::list for vSendMsg

std::deque is indexed internally, which gives some unnecessary overhead
when removing the front element.

src/net.h

@@ -770,7 +770,7 @@ public:
     size_t nSendSize; // total size of all vSendMsg entries
     size_t nSendOffset; // offset inside the first vSendMsg already sent
     uint64_t nSendBytes;
-    std::deque<std::vector<unsigned char>> vSendMsg;
+    std::list<std::vector<unsigned char>> vSendMsg;
     CCriticalSection cs_vSend;
     CCriticalSection cs_hSocket;
     CCriticalSection cs_vRecv;