Encrypt all notifications on the keyserver for iOS devices

Summary:
This differential enables encrypted notifications coalescing on the keyserver. Based on the code version of the client it is decided whether coalescible
notifications is going to be encrypted or not.

Test Plan:

1. Build iOS app.
2. Open mariadb shell and execute select * from olm_sessions;. Remember the version column value of the last session.
3. Send a couple of collapsible notifications.
4. Execute select * from olm_sessions;. Ensure last session version column value didn't change.
5. Update FUTURE_CODE_VERSION to 0.
6. Send a couple of collapsible notifications. Ensure that coalescing takes place.
7. Execute select * from olm_sessions;. Ensure last session version column value increased by the number of sent notifications.

Reviewers: tomek, atul, bartek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9149