Improve performance of PerformTask method
Improve performance of PerformTask method

1. Use a client optimised for many write operations.
2. Don't use the UpsertDocument method as it makes extra requests to cosmos to
   check if the db and collection exist, which by the time this method is called
   can be guaranteed.
3. Run upsert tasks in parallel.

BudSystemLimited/Cake.DocumentDb