Add scheduler subsystem

SiteWhere does not have the concept of scheduling actions to happen at a future time or at a recurring time/interval. The feature can be applied for many different aspects of the system including command invocations, batch operations, cleanup operations, etc. The implementation should account for:

• Cron-style scheduling expressions

• Scheduling tasks on an interval

• Scheduling tasks for one-time fire in the future

• Persistence of schedules in the datastore

• User interface for defining schedules

• Updates to allow command invocations to be scheduled