Add __slots__ to _Frame class for memory optimization

- Add __slots__ definition to _Frame class in connection.py
- Optimizes memory usage for protocol frame objects
- _Frame is created for every protocol message, making this a high-impact optimization
- Reduces object overhead from ~300+ bytes to ~40-60 bytes per frame
- All existing functionality preserved, comprehensive tests pass

Signed-off-by: Yaniv Kaul <yaniv.kaul@scylladb.com>

Author: mykaul

cassandra/connection.py

@@ -370,6 +370,8 @@ def __repr__(self):
 
 
 class _Frame(object):
+    __slots__ = ('version', 'flags', 'stream', 'opcode', 'body_offset', 'end_pos')
+
     def __init__(self, version, flags, stream, opcode, body_offset, end_pos):
         self.version = version
         self.flags = flags