thread joining is not preserved when reindexing

the current thread joining just adds references to the index entries. that's fine but if the messages are reindexed, the references obviously won't be there. i think we need to add some separate blob of information somewhere that maintains these references, which sup-sync is aware of.

if we're going down the bdb route for state preservation, that might be an obvious place to put this too, because it's essentially a hashtable keyed on message ids.