In previous, when using file scan node(eq, querying hive table), the max number of scanner for each scan node
will be the `doris_scanner_thread_pool_thread_num`(default is 48).
And if the query parallelism is N, the total number of scanner would be 48 * N, which is too many.
In this PR, I change the logic, the max number of scanner for each scan node
will be the `doris_scanner_thread_pool_thread_num / query parallelism`. So that the total number of scanners
will be up to `doris_scanner_thread_pool_thread_num`.
Reduce the number of scanner can significantly reduce the memory usage of query.
