Support optional argument for dropping stats in the truncate_table() of HMS

Certain clients like Impala rely on truncate_table() API in HMS to take care of truncate operations for replicated tables in Impala. There is an option 'set DELETE_STATS_IN_TRUNCATE=0;' to specify not to delete stats during the truncate operation. It would be good to support this optional argument whether to delete stats or not during the truncate table operation.