refactor: move crontab writing code to scheduler-docker-local plugin

The implementation was very heavily docker-local specific and thus didn't make a ton of sense to keep in the cron plugin.

This also re-introduces changes from #5908 to parallelize the crontab generation (which is also specific to the docker-local scheduler) that were lost during a rebase.

The impetus for this change was implementing a `cron:run` command. The logic for actually running the command isn't scheduler specific - merely invoking `dokku run` with the appropriate arguments - but all the logic around how the commands were retrieved were, so the change was made pre-emptively.

plugins/cron/src/triggers/triggers.go

@@ -1,39 +0,0 @@
-package main
-
-import (
-	"flag"
-	"fmt"
-	"os"
-	"strings"
-
-	"github.com/dokku/dokku/plugins/common"
-	cron "github.com/dokku/dokku/plugins/cron"
-)
-
-// main entrypoint to all triggers
-func main() {
-	parts := strings.Split(os.Args[0], "/")
-	trigger := parts[len(parts)-1]
-	flag.Parse()
-
-	var err error
-	switch trigger {
-	case "cron-write":
-		err = cron.TriggerCronWrite()
-	case "install":
-		err = cron.TriggerInstall()
-	case "post-delete":
-		err = cron.TriggerPostDelete()
-	case "post-deploy":
-		err = cron.TriggerPostDeploy()
-	case "report":
-		appName := flag.Arg(0)
-		err = cron.ReportSingleApp(appName, "", "")
-	default:
-		err = fmt.Errorf("Invalid plugin trigger call: %s", trigger)
-	}
-
-	if err != nil {
-		common.LogFailWithError(err)
-	}
-}