[Python] configurable limit of simultaneous connections (python/asyncio)

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Description of the PR

A current generated asyncio client limits number of simultaneous connections to 4 and this values is not configurable by the configuration object. In PR I use value from configuration and set default to 100 (it's default value recommended by the aiohttp library).

Thanks for reviewing and merging it :)

CC: @wing328 @taxpon (2017/07) @frol (2017/07) @mbohlool (2017/07) @cbornet (2017/09) @kenjones-cisco (2017/11) @Jyhess (2019/01)