* added model and storage layer for task
* added create task api
* added api to list tasks
* modified task proto and fixed nits
* renamed variable
* fixed ut
* fixed UT
* added UT for api_converter and resource manager
* added UT for api_converter and resource manager
* fixed BE UT
* added task storage layer UT
* changed UT
* fixed foreign key typo
* fixed typo and added comments