identity: don't queue two temporary accounts for one identity

There are cases where more than one temporary g-o-a account can get
created on kinit. That is because creating a temporary account is
asynchronous, and kinit can trigger multiple refreshes of the credential
cache in one go.

This commit accounts for all pending temporary account additions in a
hash table, and filters out duplicates.

https://bugzilla.gnome.org/show_bug.cgi?id=709103