BUG: Stop addition of new list in orders_by_modified on every dt.
BUG: Stop addition of new list in orders_by_modified on every dt.

Remove use of defaultdict for orders_by_modified, which was causing an
empty list to be added every time to_dict was called with a specified
dt.

Nnoticed in the minute emission case when hunting another memory leak,
every simulation minute a new Timestamp and list was created and never
let go.

MonoCloud/zipline,keir-rex/zipline,florentchandelier/zipline,sketchytechky/zipline,cmorgan/zipline,keir-rex/zipline,iamkingmaker/zipline,nborggren/zipline,aajtodd/zipline,Scapogo/zipline,semio/zipline,zhoulingjun/zipline,aajtodd/zipline,iamkingmaker/zipline,dkushner/zipline,dmitriz/zipline,otmaneJai/Zipline,gwulfs/zipline,ChinaQuants/zipline,michaeljohnbennett/zipline,michaeljohnbennett/zipline,stkubr/zipline,jimgoo/zipline-fork,YuepengGuo/zipline,jimgoo/zipline-fork,quantopian/zipline,humdings/zipline,stkubr/zipline,Scapogo/zipline,wubr2000/zipline,morrisonwudi/zipline,nborggren/zipline,chrjxj/zipline,semio/zipline,euri10/zipline,florentchandelier/zipline,magne-max/zipline-ja,StratsOn/zipline,joequant/zipline,grundgruen/zipline,umuzungu/zipline,ChinaQuants/zipline,joequant/zipline,jordancheah/zipline,CDSFinance/zipline,DVegaCapital/zipline,grundgruen/zipline,alphaBenj/zipline,bartosh/zipline,quantopian/zipline,kmather73/zipline,otmaneJai/Zipline,gwulfs/zipline,alphaBenj/zipline,dmitriz/zipline,bartosh/zipline,StratsOn/zipline,ronalcc/zipline,umuzungu/zipline,sketchytechky/zipline,wubr2000/zipline,MonoCloud/zipline,euri10/zipline,enigmampc/catalyst,jordancheah/zipline,wilsonkichoi/zipline,YuepengGuo/zipline,ronalcc/zipline,kmather73/zipline,wilsonkichoi/zipline,zhoulingjun/zipline,DVegaCapital/zipline,AlirezaShahabi/zipline,magne-max/zipline-ja,CDSFinance/zipline,chrjxj/zipline,cmorgan/zipline,AlirezaShahabi/zipline,dkushner/zipline,humdings/zipline,morrisonwudi/zipline,enigmampc/catalyst