Previously ByteBuffer::grow() behaved like Vector<T>::resize().
However the function name was somewhat ambiguous - and so this patch
updates ByteBuffer to behave more like Vector<T> by replacing grow()
with resize() and adding an ensure_capacity() method.
This also lets the user change the buffer's capacity without affecting
the size which was not previously possible.
Additionally this patch makes the capacity() method public (again).
The old enumeration didn't allow discriminating the key exchange
algorithms used, but only allowed the handshake with the server. With
this new enumeration, we can know which key exchange algorithm we are
actually supposed to use :^)
