Go client connection initialisation to an empty event stream is slow

[h3yduck]

Nomad version

Nomad v1.1.0 (2678c36)

Operating system and Environment details

• most probably affects all operating systems (tested on macOS Big Sur and debian:buster Docker image)
• most probably affects all client libraries where the HTTP client waits for any response (at least HTTP headers) before returning the HTTP body to the caller (tested on go version go1.16 darwin/amd64 and go version go1.16.2 linux/amd64)

Issue

Go client connection initialisation to an empty event stream takes 30 seconds: client waits until first heartbeat before returning.

Reproduction steps

1. Start Nomad in dev mode with nomad agent -dev
2. Connect to Nomad event stream's job topic having 0 Nomad jobs with time go run .:

   package main
   
   import (
   	"context"
   
   	"github.com/hashicorp/nomad/api"
   )
   
   func main() {
   	nomadClient, _ := api.NewClient(&api.Config{})
   	eventStreamClient := nomadClient.EventStream()
   	topics := map[api.Topic][]string{
   		api.TopicJob: {"*"},
   	}
   	eventStreamClient.Stream(context.Background(), topics, 0, nil)
   }

Expected Result

eventStreamClient.Stream call returns in a reasonable amount of time (< 1s).

Actual Result

eventStreamClient.Stream call returns after 30s.

[github-actions]

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