CLOUDSTACK-6740: Direct agent command throttling improvements
List of changes:
1. Created a separate thread pool for handling cron and ping tasks. The size of the pool is based on direct.agent.pool.size. The existing direct agent pool will run all commands other than cron and ping.
2. For normal tasks (generated as part of user/admin API calls), if throttle limit is reached then tasks get queued up for subsequent execution once threads are available.
3. For cron and ping tasks (internally generated by MS like ping, VM sync etc.), if throttle limit is reached then these gets rejected. Since these are internally generated these can be rejected without any issues.