[improvement](scan) avoid too many scanners for file scan node (#25727)

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.

be/src/vec/exec/vmysql_scan_node.cpp

@@ -227,7 +227,8 @@ void VMysqlScanNode::debug_string(int indentation_level, std::stringstream* out)
     }
 }
 
-Status VMysqlScanNode::set_scan_ranges(const std::vector<TScanRangeParams>& scan_ranges) {
+Status VMysqlScanNode::set_scan_ranges(RuntimeState* state,
+                                       const std::vector<TScanRangeParams>& scan_ranges) {
     return Status::OK();
 }
-} // namespace doris::vectorized
+} // namespace doris::vectorized