Python: Add ASGI lifespan state support.

Lifespan state is a special dict in asgi lifespan scope, which allow
applications to persist data from the lifespan cycle to request/response
handling. The s

Python: Add ASGI lifespan state support.

Lifespan state is a special dict in asgi lifespan scope, which allow
applications to persist data from the lifespan cycle to request/response
handling. The scope["state"] namespace provides a place to store these
sorts of things. The server will ensure that a shallow copy of the
namespace is passed into each subsequent request/response call into the
application.

Some frameworks are already taking advantage of this feature, for
example, starlette, and without this feature they wouldn't work
properly.

Signed-off-by: synodriver <diguohuangjiajinweijun@gmail.com>
Reviewed-by: Andrew Clayton <a.clayton@nginx.com>
[ Minor code tweaks to avoid lines > 80 chars, static a function and
re-work the PyMemberDef structure initialisation for Python <3.7
and -Wwrite-strings compatibility - Andrew ]
Tested-by: <https://github.com/synodriver>
Tested-by: <https://github.com/hawiliali>
Closes: <https://github.com/nginx/unit/issues/864>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

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Python: request processing in multiple threads.

This closes #459 issue on GitHub.

Python: ASGI server introduced.

This closes #461 issue on GitHub.