v12.0.0

• Improved exampless (#870)
• Fixed example broker's invalid sync connack call (#869)
• Added no CONNACK sending mode to the example broker (#869)
• Added posting code to acceptors (#867)
• Added io_context and threads setting options for example broker (#866, #871, #872)
• Fixed moved from object access (#865)
• <<<< breaking change >>>> Removed ioc_con() and ioc_accept() (#866)
• <<<< breaking change >>>> Unified automatic async/sync choosing flag. (#857, #862)
  endppoint constructor 4th parameter async_operation is used.
  set_async_operation() can overwrite the async_operation.
  It should be called before sending any packets.
• Implemented clean shutdown (WS TLS TCP) (#849, 860, #863, #864)
• Improved build system (#859)
• Added DISCONNECT with session taken over reason code (#847)
• Improved tests (#845, #846, #852)
• Fixed invalid maximum_qos (2) sending (#844)
• Added multi thread support for example broker (#842, #855, #866)
• Replaced example broker's API call from sync to async (#842)
• Replaced use_certificate_file() with use_certificate_chain_file() to support both server cert and server - intermediate CA cert (#841)
• <<<< breaking change >>>> Added async_force_disconnect(). force_disconnect() is removed from async_client (#840, #868)
• Added will delay interval support (#839)
• Fixed async_handler_t calling timing (#836)
• Added BOOST_ASIO_NO_TS_EXECUTORS support (#830)
• Fixed async_unsuback() argument mismatch (#828)
• Added receive maximum support(#825, #835)
• Fixed QoS2 packet handling problem (#824)
• Refined async test mechanism. MQTT_ORDERED() macro is introcuded (#822, #823)
• Added maximum packet size support (#821, #826, #834))