net/http: keep idle conns sorted by usage
net/http: keep idle conns sorted by usage

Addressing feedback from Alan Su in https://golang.org/cl/22655

Change-Id: Ie0724efea2b4da67503c074e265ec7f8d7de7791
Reviewed-on: https://go-review.googlesource.com/22709
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

cloudflare/go,suncycheng/go,momchil-velikov/go,akutz/go,codestation/go,cloudflare/go,derekmarcotte/go,akutz/go,nawawi/go,cloudflare/go,ganboing/go-esx,akutz/go,suncycheng/go,cloudflare/go,derekmarcotte/go,CAFxX/go,akutz/go,josharian/go,nawawi/go,ganboing/go-esx,goodlang/good,vsekhar/elastic-go,ImJasonH/go,ImJasonH/go,cloudflare/go,josharian/go,momchil-velikov/go,codestation/go,goodlang/good,polyverse-security/go,codestation/go,nawawi/go,4ad/go,derekmarcotte/go,ImJasonH/go,theif519/go,ImJasonH/go,josharian/go,ganboing/go-esx,ganboing/go-esx,CAFxX/go,suncycheng/go,momchil-velikov/go,linux-on-ibm-z/go,polyverse-security/go,linux-on-ibm-z/go,theif519/go,momchil-velikov/go,theif519/go,codestation/go,Cofyc/go,josharian/go,momchil-velikov/go,vsekhar/elastic-go,nawawi/go,momchil-velikov/go,derekmarcotte/go,ganboing/go-esx,4ad/go,Cofyc/go,4ad/go,vsekhar/elastic-go,polyverse-security/go,4ad/go,codestation/go,akutz/go,theif519/go,josharian/go,goodlang/good,akutz/go,Cofyc/go,derekmarcotte/go,Cofyc/go,momchil-velikov/go,cloudflare/go,akutz/go,golang/go,vsekhar/elastic-go,Cofyc/go,goodlang/good,nawawi/go,4ad/go,ImJasonH/go,polyverse-security/go,momchil-velikov/go,CAFxX/go,4ad/go,ImJasonH/go,CAFxX/go,ganboing/go-esx,codestation/go,theif519/go,nawawi/go,linux-on-ibm-z/go,goodlang/good,goodlang/good,linux-on-ibm-z/go,golang/go,golang/go,suncycheng/go,goodlang/good,akutz/go,josharian/go,linux-on-ibm-z/go,golang/go,golang/go,4ad/go,theif519/go,nawawi/go,CAFxX/go,polyverse-security/go,codestation/go,josharian/go,Cofyc/go,ganboing/go-esx,linux-on-ibm-z/go,derekmarcotte/go,codestation/go,CAFxX/go,theif519/go,CAFxX/go,goodlang/good,linux-on-ibm-z/go,vsekhar/elastic-go,derekmarcotte/go,derekmarcotte/go,nawawi/go,polyverse-security/go,vsekhar/elastic-go,CAFxX/go,josharian/go,theif519/go,suncycheng/go,cloudflare/go,Cofyc/go,christopher-henderson/Go,ImJasonH/go,vsekhar/elastic-go,polyverse-security/go,4ad/go,suncycheng/go,golang/go,vsekhar/elastic-go,cloudflare/go,golang/go,ganboing/go-esx,golang/go,linux-on-ibm-z/go,suncycheng/go,ImJasonH/go,suncycheng/go,polyverse-security/go,Cofyc/go