Fix bug in SparkLoadTask for `CACHE <partitioned_table>`

Each partition would result in a new partitioned table entry ...
Allow users to control the number of tasks for table scan operations
This will enable user's queries to perform well even for high cardinality partitioned tables
Merge pull request #305 from sundeepn/coalesce

Allow users to control the number of tasks for table scan operations
Merge pull request #296 from harveyfeng/master

Fix bug in SparkLoadTask for `CACHE <partitioned_table>`