Schedule repodata after cloning patches

This fixes a bug where repository metadata, in particular patch data, would not be updated after a channel is cloned. Currently `ChannelManager.queueChannelChange()` is called after a channel is cloned with its packages, but typically before errata cloning is finished (this happens in the "background" action `CloneErrataAction`, which usually takes longer).

We also added a unit test with basic coverage of `CloneErrataAction`.

d6457b1 fixes a related issue where, at least on Postgres databases, timestamps would be returned with millisecond precision making the comparison fail.