Create request_id directive #1590 (#1711)

* Create request_id directive #1590

* Address Comments

* Fix TestListenerAddrEqual

* requestid: Add some tests

* Address Comments by tobya

* Address Comments

caddyhttp/httpserver/plugin.go

@@ -443,6 +443,7 @@ var directives = []string{
 	// services/utilities, or other directives that don't necessarily inject handlers
 	"startup",
 	"shutdown",
+	"requestid",
 	"realip", // github.com/captncraig/caddy-realip
 	"git",    // github.com/abiosoft/caddy-git