[Tunnelbroker] update DDB schema to handle client message ID

Summary: Issue and context why we need this: ENG-5171.

Test Plan:

1. Send some messages and check if they're properly saved in DB:

2. Run test_messages_order to make sure:
3. messages can be properly retrieved and deleted
4. order is correct

Reviewers: varun, michal, bartek, jon

Reviewed By: michal, bartek

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9459