The subprotocol shall use binary frames, and AO protocol stays separated
within the text frames.

Introduce the subprotocol ("Einsof"), its prototype serialization and
parsing functions, and its first set of messages.

These messages are carriers of public-key authentication mechanism which
involves client request, server challenge, and client response. An
"ident" message is used to tell a compatible server that you support a
particular version of the subprotocol.

Note: the functions that handle encoding are very specialized.
They're not representative of how the wire format should be generally
handled, and were written this way because the first set of messages is
tiny and simple enough.

* Remove TCP entry point

Resolve #987

* Remove TCP entry point
  * Servers that do not support WebSocket will be marked as `Legacy`
  * Removal of TCP connection from the master will follow later.

* Tweaked error message