Never build a session with oneself

If we detect our own ID is not in our own devicelist on receiving an
update, we reannounce ourselves. This used to have the side effect of
modifying the list of devices we thought were in the update set, causing
us to accidentally build a session with ourselves.

This lead to our own key being set to TRUSTED_INACTIVE, resulting in red
lock icons on messages sent by the own device.

We fix this by having publishOwnDeviceId() operate on a copy of the
original set. This commit also includes a db migration which deletes
sessions with oneself and sets own keys back to TRUSTED.