After fixing ngx_http_v3_encode_varlen_int() in 400eb1b628,
NGX_HTTP_V3_VARLEN_INT_LEN retained the old value of 4, which is
insufficient for the values over 1073741823 (1G - 1).

The NGX_HTTP_V3_VARLEN_INT_LEN macro is used in ngx_http_v3_uni.c to
format stream and frame types.  Old buffer size is enough for formatting
this data.  Also, the macro is used in ngx_http_v3_filter_module.c to
format output chunks and trailers.  Considering output_buffers and
proxy_buffer_size are below 1G in all realistic scenarios, the old buffer
size is enough here as well.

Now the session object is assigned to c->data while ngx_http_connection_t
object is referenced by its http_connection field, similar to
ngx_http_v2_connection_t and ngx_http_request_t.

The change allows to eliminate v3_session field from ngx_http_connection_t.
The field was under NGX_HTTP_V3 macro, which was a source of binary
compatibility problems when nginx/module is build with/without HTTP/3 support.

Postponing is essential since c->data should retain the reference to
ngx_http_connection_t object throughout QUIC handshake, because SSL callbacks
ngx_http_ssl_servername() and ngx_http_ssl_alpn_select() rely on this.

Now "listen" directve has a new "quic" parameter which enables QUIC protocol
for the address.  Further, to enable HTTP/3, a new directive "http3" is
introduced.  The hq-interop protocol is enabled by "http3_hq" as before.
Now application protocol is chosen by ALPN.

Previously used "http3" parameter of "listen" is deprecated.

It's called after handshake completion or prior to the first early data stream
creation.  The callback should initialize application-level data before
creating streams.

HTTP/3 callback implementation sets keepalive timer and sends SETTINGS.

Also, this allows to limit max handshake time in ngx_http_v3_init_stream().

ngx_http_v3_init() is renamed ngx_http_v3_init_stream().
ngx_http_v3_reset_connection() is renamed to ngx_http_v3_reset_stream().

Now main QUIC connection for HTTP/3 always has c->idle flag set.  This allows
the connection to receive worker shutdown notification.  It is passed to
application level via a new conf->shutdown() callback.

The HTTP/3 shutdown callback sends GOAWAY to client and gracefully shuts down
the QUIC connection.

The change removes hq-specific request handler.  Now hq requests are handled
by the HTTP/3 request handler.