Close all connections on exit (Phase 1)

When an example application exits, iterate over all client
and server connections and send a CONNECTION_CLOSE message
on each before cleaning up all resources.

This is only a simple initial implementation (Phase 1)
that will successfully close a small number of connections.
Phase 2 will deal with larger numbers of connections and
the load that closing these will place on a single UDP
socket.

Right now, we don't have complete ACK frame type in
NGTCP2_ERR_ACK_FRAME because it includes variable flags, and cannot
use specific error code range (0x800001xx).

To fill flags for stream and ack, flags field is filled when encoding
them.