QUIC: refined the "c->quic->initialized" flag usage.

QUIC: refined the "c->quic->initialized" flag usage.

The flag is tied to the initial secret creation.  The presence of c->quic
pointer is sufficient to enable execution of ngx_quic_close_quic().

The ngx_quic_new_connection() function now returns the allocated quic
connection object and the c->quic pointer is set by the caller.

If an early error occurs before secrets initialization (i.e. in cases
of invalid retry token or nginx exiting), it is still possible to
generate an error response by trying to initialize secrets directly
in the ngx_quic_send_cc() function.

Before the change such early errors failed to send proper connection close
message and logged an error.

An auxilliary ngx_quic_init_secrets() function is introduced to avoid
verbose call to ngx_quic_set_initial_secret() requiring local variable.