migrate player_caches to scylla

[howardchung]

Eventually I think we can migrate everything over, but this will give us a way to start testing. There is also some corrupted data in some of the player_matches tables which may complicate a full migration.

• Set up a Scylla cluster
• Add code to double-write the player caches
• Add code/config to read player_caches from scylla
• Write a migration service to transfer data over from cassandra to scylla
• Delete the old player caches code
• Drop the player_caches from cassandra
• Resize/reduce Cassandra nodes