QUIC: prevented generating ACK frames with discarded keys.

QUIC: prevented generating ACK frames with discarded keys.

Previously it was possible to generate ACK frames using formally discarded
protection keys, in particular, when acknowledging a client Handshake packet
used to complete the TLS handshake and to discard handshake protection keys.
As it happens late in packet processing, it could be possible to generate ACK
frames after the keys were already discarded.

ACK frames are generated from ngx_quic_ack_packet(), either using a posted
push event, which envolves ngx_quic_generate_ack() as a part of the final
packet assembling, or directly in ngx_quic_ack_packet(), such as when there
is no room to add a new ACK range or when the received packet is out of order.
The added keys availability check is used to avoid generating late ACK frames
in both cases.