Remove use of async_callback (cf. https://github.com/tornadoweb/tornado/commit/1d1b0dec)
Remove use of async_callback (cf. https://github.com/tornadoweb/tornado/commit/1d1b0dec)

tellapart/flower,ChinaQuants/flower,raphaelmerx/flower,getupcloud/flower,pygeek/flower,lucius-feng/flower,barseghyanartur/flower,pj/flower,pygeek/flower,asmodehn/flower,barseghyanartur/flower,Lingling7/flower,ucb-bar/bar-crawl-web,asmodehn/flower,ucb-bar/bar-crawl-web,asmodehn/flower,pj/flower,lucius-feng/flower,getupcloud/flower,raphaelmerx/flower,alexmojaki/flower,allengaller/flower,Lingling7/flower,ChinaQuants/flower,jzhou77/flower,raphaelmerx/flower,pygeek/flower,lucius-feng/flower,pj/flower,Lingling7/flower,getupcloud/flower,jzhou77/flower,alexmojaki/flower,ChinaQuants/flower,allengaller/flower,barseghyanartur/flower,marrybird/flower,tellapart/flower,allengaller/flower,marrybird/flower,jzhou77/flower,alexmojaki/flower,marrybird/flower,tellapart/flower,ucb-bar/bar-crawl-web