Add SyncMetadata.ID

[stephencelis]

We can use a multi-column @Selection to represent SyncMetadata's composite primary key, which provides a more straightforward API for joining into the SyncMetadata table:

RemindersList
  .leftJoin(SyncMetadata.all) { $0.syncMetadataID.eq($1.id) }

This also provides a straightforward API for fetching sync metadata for a given ID:

SyncMetadata.find(reminder.syncMetadataID)