SharedSyncLog cleanup function

[ShishKabab]

To synchronize between clients, a SharedSyncLog is used to distribute log entries. Once all devices have sent and received all log entries up to a certain point, they can be deleted.

To implement:

• Add a doPeriodicCleanup() : Promise<void> method to the SharedSyncLog interface
• In SharedSharedSyncLogStorage.doPeriodicCleanup(), retrieve the device info for the devices of the current user
• Get the minimum of the sharedUntil values of those devices.
• Delete all log entries before that value