bert_encode() not thread-safe

[WayneCao]

In server.cpp, bert_encode seems not thread-safe, different invocation works in same memory buffer in bert_context

while(true) {
    std::string string_in = receive_string(new_socket);
    if (string_in.empty()) {
        break;
    }
    std::vector<float> embeddings = std::vector<float>(n_embd);
    bert_encode(bctx, params.n_threads, string_in.data(), embeddings.data());
    send_floats(new_socket, embeddings);
}