Add beast::asio::basic_streambuf:

This is class whose interface is identical to the `boost::asio::basic_streambuf`, and uses an implementation that stores the data in multiple discontiguous linear buffers, expanding and shrinking as needed. The implementation is complex, with the core algorithm implemented in these functions `prepare`, `commit`, and `consume`. These functions need to be audited line by line for correctness. The Overlay peers are modified to use this new streambuf (its a drop in replacement for `boost::asio::basic_streambuf`). rippled runs connected to the network without assertions or crashes using this code.
Reviewers: @rec @JoelKatz @ximinez

I don't understand what you are asking. Would you rather see:
```
    if (! (block_size > 0))
    {
        throw std::invalid_argument(
            "basic_streambuf: invalid block_size");
    }
```