HTTP/3: avoid recreation of standard client uni streams

HTTP/3: avoid recreation of standard client uni streams

Creating a control/encoder/decoder stream while another such stream
already exists, is not allowed.  Also, closing such a stream results
in connection closure with NGX_HTTP_V3_ERR_CLOSED_CRITICAL_STREAM.
However, since stream creation and connection closure are asynchronous,
there could be a window where two control/encoder/decoder streams
could coexist within a single cycle iteration.  This could result
in reusing parsing context, such as encoder insert buffer.

The change adds a mask for all standard client uni streams ever created.
This allows to check if a stream of this type was created before.
While here, mandatory stream validation now also uses this mask.