Simplify the way OpenSSL renegotiation is initiated in server.

Simplify the way OpenSSL renegotiation is initiated in server.

At least in all modern versions of OpenSSL, it is enough to call
SSL_renegotiate() once, and then forget about it. Subsequent SSL_write()
and SSL_read() calls will finish the handshake.

The SSL_set_session_id_context() call is unnecessary too. We only have
one SSL context, and the SSL session was created with that to begin with.