Ignore incoming sync records for devices we don't know about.

[mhammond]

This bit us in desktop - it would have bitten us much less if this was fixed.

If we don't know the device we should ignore the record - so this branch changes to, say a log::warn about not storing, then a continue to ignore the record.

┆Issue is synchronized with this Jira Task

[mhammond]

Turns out we can't actually do this because stand alone syncing (ie, store.sync()) doesn't allow us to pass the device list in, so we'd get no rows! :( I'm mildly surprised iOS tab syncing doesn't seem to be bitten by this, which I suspect is going to come back and bite us moving iOS to the sync manager.

(I'm keeping it open because I think we should still do this once we kill stand-alone syncing)

[mhammond]

I think we already did this.