* refactor: move handshake_done args into a convenience struct
* refactor: move peer_id from peerIo to tr_handshake
tr_handshake is a short-term object and tr_peerIo is long-term, so this
effectively narrows the scope of this field.
* chore: remove unused field tr_peerIo.isEncrypted
* refactor: add tr_peer_id_t type to hold peer ids.
this is a 'using' alias to a std::array<> so that code passing peer-ids
around doesn't have to memcmp / memcpy PEER_ID_LEN anymore. Also removes
the now-unused PEER_ID_LEN macro.
