websocket: Change some Future type annotations to Awaitables
websocket: Change some Future type annotations to Awaitables

This conforms with usage elsewhere, that methods returning None use
Futures (self-starting in case the result is not awaited) and others
use Awaitable (reserving the option to use more efficient native
coroutines).

dongpinglai/my_tornado,dongpinglai/my_tornado,bdarnell/tornado,dongpinglai/my_tornado,allenl203/tornado,wujuguang/tornado,lilydjwg/tornado,NoyaInRain/tornado,bdarnell/tornado,mivade/tornado,mivade/tornado,NoyaInRain/tornado,NoyaInRain/tornado,allenl203/tornado,lilydjwg/tornado,wujuguang/tornado,wujuguang/tornado,NoyaInRain/tornado,mivade/tornado,tornadoweb/tornado,allenl203/tornado,mivade/tornado,bdarnell/tornado,lilydjwg/tornado,mivade/tornado,wujuguang/tornado,wujuguang/tornado,NoyaInRain/tornado,dongpinglai/my_tornado,lilydjwg/tornado,bdarnell/tornado,allenl203/tornado,tornadoweb/tornado,tornadoweb/tornado,NoyaInRain/tornado,allenl203/tornado,bdarnell/tornado,dongpinglai/my_tornado,tornadoweb/tornado,dongpinglai/my_tornado