This reverts commit af500a4b and applies
a different fix. The problem was that persona changes could get away without
being notified when disconnecting due to an account being disabled. This
is because, after storing the cache, an additional check is performed for
whether the account was enabled. If it is, the cache is loaded (and persona
changes notified). If not, the cache is not loaded, *and persona changes
were not notified*.

This situation could occur when a Telepathy account is disabled:
 1. _notify_connection_cb() is called, but the TpAccount::enabled property
    is still true.
 2. While in store_cache(), the TpAccount::enabled property goes false.
 3. The check before load_cache() would fail, so load_cache() would never
    be called.

Similarly, it could occur when a Telepathy account is removed, with
TpAccountManager:account-removed being emitted while in store_cache().

Closes: https://bugzilla.gnome.org/show_bug.cgi?id=683390