Generic, framing-agnostic connection serve loop.

Each protocol handler is a module implementing the FakeRiak.Protocol behaviour. serve/2 owns the socket, buffers incoming bytes, and repeatedly asks the handler to parse the next complete request out of the buffer.

The handler stays purely functional over the accumulated buffer: it never touches the socket directly and never blocks waiting for more bytes. When the buffer does not yet hold a full request it returns {:need_more, state} and this loop takes care of reading more data.

A connection whose buffer grows past 16 MiB without ever containing a complete request is dropped, so a runaway client cannot balloon memory.

A handler on a plain-TCP connection can also request an in-band TLS upgrade ({:upgrade_tls, ...}, used for Riak's RpbStartTls): the loop sends the acknowledgement in plaintext, terminates TLS on the same socket, and carries the handler state over to the encrypted stream.