Better mirror handling and config with AQT_CONFIG

Fixed

• Refactoring redirect helper function to improve connection error checks and error message.(#109)
• Detect exception on each download and extraction threads.
• Race condition error happend on py7zr. require py7zr>=0.5.3.(#97)
• Work around for http://download.qt.io/ returns wrong metalink xml data.(#105, #106)

Changed

• Support specifying config with environment variable AQT_CONFIG
• Improve error messages when command argument is wrong.
• Allow path search for 7z (#96)
• Simplify multi-threading using concurrent.futures.ThreadPoolExecutor().