added ability to prune scheduled jobs and triggers on startup

this patch adds the ability to prune scheduled jobs and triggers on startup

reason:
if you add and remove jobs and/or triggers in a project from time to time, you will have leftover scheduled jobs and triggers in redis, which doesn't hurt, but it's not nice either.
this patch adds a knob which, when enabled, will prune all scheduled jobs and triggers on startup

the default value is false

to enable this setting, add the following to your Config.groovy:
```
grails {
    jesque {
        pruneScheduledJobsOnStartup = true
    }
}
```