The previous commit added more generic APIs for handling
NXT_CONF_VALUE_ARRAY and non-NXT_CONF_VALUE_ARRAY together.
Modify calling code to remove special cases for arrays and
non-arrays, taking special care that the path for non arrays is
logically equivalent to the previous special cased code.
Use the now-generic array code only.

Introduced in the 78864c9d5ba8 commit.

Sorry about that.

The __call__ method can be native and not be a PyFunction type.  A type check
is thus required before accessing op_code and other fields.

Reproduced on Ubuntu 21.04, Python 3.9.4 and Falcon framework: here, the
App.__call__ method is compiled with Cython, so accessing op_code->co_flags is
invalid; accidentally, the COROUTINE bit is set which forces the Python module
into the ASGI mode.

The workaround is explicit protocol specification.

Note: it is impossible to specify the legacy mode for ASGI.

Introducting application graceful stop.  For now only used when application
process reach request limit value.

This closes #585 issue on GitHub.

Unit's ASGI implementation creates a new event loop to run an application for
each thread since 542b5b8c0647.  This may cause unexpected exceptions or
strange bugs if asyncio synchronisation primitives are initialised before the
application starts (e.g. globally).

Although the approach with a new event loop for the main thread is consistent
and helps to prepare the application to run in multiple threads, it can be a
source of pain for people who just want to run single-threaded ASGI
applications in Unit.

This is related to #560 issue on GitHub.

An ASGI application can cancel the Future object returned by the receive()
call.  In this case, Unit's ASGI implementation should not call set_result()
because the Future is already handled.  In particular, the Starlette framework
was noted to cancel the received Future.

This patch adds a done() check for the Future before attempting a set_result().

This is related to #564 issue on GitHub.

The receive() call never blocks for a GET request and always returns the same
empty body message.  The Starlette framework creates a separate task when
receive() is called in a loop until an 'http.disconnect' message is received.

The 'http.disconnect' message was previously issued after the response header
had been sent.  However, the correct behavior is to respond with
'http.disconnect' after sending the response is complete.

This closes #564 issue on GitHub.