Ensure that we use constants everywhere (#7845)

This allows for canonicalization of the strings
throughout our code and provides a common space
for all these constants to reside.

This list is rather non-exhaustive but captures
all the headers used in AWS S3 API operations

cmd/peer-rest-server.go

@@ -28,6 +28,7 @@ import (
 	"time"
 
 	"github.com/gorilla/mux"
+	xhttp "github.com/minio/minio/cmd/http"
 	"github.com/minio/minio/cmd/logger"
 	"github.com/minio/minio/pkg/event"
 	xnet "github.com/minio/minio/pkg/net"
@@ -675,7 +676,7 @@ func (s *peerRESTServer) TraceHandler(w http.ResponseWriter, r *http.Request) {
 	}
 	trcAll := r.URL.Query().Get(peerRESTTraceAll) == "true"
 
-	w.Header().Set("Connection", "close")
+	w.Header().Set(xhttp.Connection, "close")
 	w.WriteHeader(http.StatusOK)
 	w.(http.Flusher).Flush()