[Issue#89] Support invocation of multiple concurrent actions

[spark0510]

1. Add Rank on the schema
2. Rank in the json would be like following: {"Rank": "1/4"} - it indicates the first action out of 4
3. faasr_check_workflow_cycle ignores the Rank
4. faasr_run_user_function leaves the .done file being adaptive to Rank: sum.1.done, sum.2.done and so on
5. faasr_abort_on_mutiple_invocation reads the Rank and aborts the action if all actions including ones with Rank haven't completed.
6. Implement faasr_rank to return Rank. Return value is a list consisting of MaxRank and Rank. Users can retrieve these values by following functions:

   rank_list <- FaaSr::faasr_rank()
   MaxRank <- rank_list$MaxRank
   Rank <- rank_list$Rank