perf: skip ColDesc namedtuple allocation when no column encryption policy

When column_encryption_policy is None (the common case), the ColDesc
namedtuple list is never accessed by unpack_plain_row(). Skip building
it entirely, passing None to ParseDesc instead.

This avoids N namedtuple allocations per query (where N = number of
columns) on the Cython fast path for non-encrypted workloads.

Author: mykaul

cassandra/row_parser.pyx

@@ -32,8 +32,9 @@ def make_recv_results_rows(ColumnParser colparser):
         self.column_names = [md[2] for md in column_metadata]
         self.column_types = [md[3] for md in column_metadata]
 
+        coldescs = [ColDesc(md[0], md[1], md[2]) for md in column_metadata] if column_encryption_policy else None
         desc = ParseDesc(self.column_names, self.column_types, column_encryption_policy,
-                        [ColDesc(md[0], md[1], md[2]) for md in column_metadata],
+                        coldescs,
                         make_deserializers(self.column_types), protocol_version)
         reader = BytesIOReader(f.read())
         try: