performance: do not zero temporary buffer

    std::vector<rpc_get_dirents_in_t> rpc_in(host_size);

    std::vector<char*> recv_buffers(host_size);

    // preallocate receiving buffer. The actual size is not known yet.

    auto recv_buff = std::make_unique<char[]>(RPC_DIRENTS_BUFF_SIZE);

    /* preallocate receiving buffer. The actual size is not known yet.

     *

     * On C++14 make_unique function also zeroes the newly allocated buffer.

     * It turns out that this operation is increadibly slow for such a big

     * buffer. Moreover we don't need a zeroed buffer here.

     */

    auto recv_buff = std::unique_ptr<char[]>(new char[RPC_DIRENTS_BUFF_SIZE]);

    const unsigned long int per_host_buff_size = RPC_DIRENTS_BUFF_SIZE / host_size;

    hg_return_t hg_ret;