extract epoch manager factory out of epoch_manager.h
extract epoch manager factory out of epoch_manager.h

PauloAmora/peloton,wangziqi2016/peloton,jessesleeping/iso_peloton,malin1993ml/peloton,apavlo/peloton,seojungmin/peloton,vittvolt/peloton,seojungmin/peloton,apavlo/peloton,ShuxinLin/peloton,apavlo/peloton,cmu-db/peloton,phisiart/peloton-p3,jessesleeping/iso_peloton,yingjunwu/peloton,malin1993ml/peloton,apavlo/peloton,phisiart/peloton-p3,yingjunwu/peloton,AllisonWang/peloton,vittvolt/15721-peloton,apavlo/peloton,haojin2/peloton,cmu-db/peloton,prashasthip/peloton,AngLi-Leon/peloton,yingjunwu/peloton,vittvolt/15721-peloton,PauloAmora/peloton,vittvolt/peloton,jessesleeping/iso_peloton,AngLi-Leon/peloton,malin1993ml/peloton,yingjunwu/peloton,phisiart/peloton-p3,wangziqi2016/peloton,haojin2/peloton,cmu-db/peloton,haojin2/peloton,prashasthip/peloton,cmu-db/peloton,ShuxinLin/peloton,vittvolt/peloton,jessesleeping/iso_peloton,seojungmin/peloton,AllisonWang/peloton,prashasthip/peloton,vittvolt/15721-peloton,PauloAmora/peloton,AngLi-Leon/peloton,vittvolt/peloton,prashasthip/peloton,AngLi-Leon/peloton,wangziqi2016/peloton,seojungmin/peloton,prashasthip/peloton,phisiart/peloton-p3,jessesleeping/iso_peloton,ShuxinLin/peloton,wangziqi2016/peloton,AllisonWang/peloton,ShuxinLin/peloton,apavlo/peloton,PauloAmora/peloton,phisiart/peloton-p3,AngLi-Leon/peloton,malin1993ml/peloton,AllisonWang/peloton,phisiart/peloton-p3,vittvolt/peloton,haojin2/peloton,ShuxinLin/peloton,yingjunwu/peloton,vittvolt/peloton,vittvolt/15721-peloton,yingjunwu/peloton,haojin2/peloton,haojin2/peloton,AllisonWang/peloton,wangziqi2016/peloton,wangziqi2016/peloton,jessesleeping/iso_peloton,prashasthip/peloton,cmu-db/peloton,malin1993ml/peloton,cmu-db/peloton,ShuxinLin/peloton,seojungmin/peloton,malin1993ml/peloton,AllisonWang/peloton,vittvolt/15721-peloton,PauloAmora/peloton,PauloAmora/peloton,seojungmin/peloton,jessesleeping/iso_peloton,vittvolt/15721-peloton,AngLi-Leon/peloton