v2/checksum: refactor to type branch in constructor
v2/checksum: refactor to type branch in constructor

Zirpon/tchannel,chenwenbin928/tchannel,Zirpon/tchannel,hustxiaoc/tchannel,benfleis/tchannel,vanloswang/tchannel,i/tchannel,sasa233/tchannel,bunnyblue/tchannel,hustxiaoc/tchannel,RyanTech/tchannel,benfleis/tchannel,chenwenbin928/tchannel,bunnyblue/tchannel,vanloswang/tchannel,sasa233/tchannel,sasa233/tchannel,RyanTech/tchannel,i/tchannel,Zirpon/tchannel,savaki/tchannel,bunnyblue/tchannel,benfleis/tchannel,hustxiaoc/tchannel,i/tchannel,RyanTech/tchannel,vanloswang/tchannel,savaki/tchannel,savaki/tchannel,chenwenbin928/tchannel