Add beast::asio::basic_streambuf (RIPD-661):

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'm not super-familiar with allocators, but I believe that `allocate` just does a memory allocation but doesn't construct an object. Isn't `e` *completely* uninitialized at this point, with its constructor not having yet run?