Moving request limit control to libunit.

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

This closes #585 issue on GitHub.

Python: using default event_loop for main thread for ASGI.

Unit's ASGI implementation creates a new event loop to run an application for
each thread since 542b5b8c0647. This may cause unexpected ex

Python: using default event_loop for main thread for ASGI.

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.

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Python: support for multiple targets.

Python: request processing in multiple threads.

This closes #459 issue on GitHub.

Python: introducting macro to simplify minor version check.

Fixed building with Python 3.9.

PyUnicode_GET_SIZE() in deprecated since 3.3 and will be removed in 3.12.
In version 3.9 it was explicitly marked by deprecation warning causing
compilation error wit

Fixed building with Python 3.9.

PyUnicode_GET_SIZE() in deprecated since 3.3 and will be removed in 3.12.
In version 3.9 it was explicitly marked by deprecation warning causing
compilation error with Unit.

PyUnicode_GET_LENGTH() must be used instead.

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Python: ASGI server introduced.

This closes #461 issue on GitHub.

Python: split module initialization from WSGI implementation.

This is required for futher ASGI implementation.