RelayRequest: factor out core peer connection selection
RelayRequest: factor out core peer connection selection

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