Improve key_responder documentation.
Remove obsolete Doxygen setting.
Make base class link_receiver hold shared_ptr<host>.

Removes some data duplication from subclasses.
Add send_probe0() function implementation.
Tricky place - this is HMAC initialization with key. Add comment.
Mark todo.
Add dh_size() helper function.
Return proper type in remote_endpoint().
Add idle state for key_initiator.
Simplify constructor signature for key_initiator.
Add hmac_verify() and hmac_append() helper functions.
Allow getting initiators by endpoint.
Rearrange, clean and document key_responder/key_initiator.