Add const qualifier for `TcpClient::isReconnect()` method.

[Guo-Shiyu]

[ithewei]

这个不加const会有什么影响吗？

[Guo-Shiyu]

In my use case, the following code will cause a compile error without compile flag -fpermissive .

void f(const hv::TcpClient* raw) { // or `const hv::TcpClient&`
    if (raw->isReconnet()) {
        ... 
    } 
    // compile error: `raw` has type `const hv::TcpClient*` but member function
    // `TcpCLient::isReconnect()` not marked as const.

}

Add const qualifier for this method will not intruduce any incompatibility to existed code. So it would be a good fix?