Do not re-import keys for sources that already have them

[eloquence]

Due to this logic, after each sync, we attempt to re-import every key for every source from the server. There is no need to re-import keys we already have, as a source can not change their keypair except by logging in as a new source.

This unnecessary re-import can cause performance issues when using the client with a large number of sources and should be resolved as part of mitigation for #716.

(Copied in part from @redshiftzero's comment here: #733 (comment))

[eloquence]

@rmol this was removed from #733 per this comment, could you create a separate PR for it? It's still worth doing for beta if it shaves off a few seconds when using >200 sources. Not release blocker level but I think we should keep it on the milestone (unless there any negative side-effects I'm not aware of).