[Taler] sync transitivity

[Jeff Burdges]

I mentioned "transitivity" in wallet linking schemes to Florian at lunch
today.  If a links with b and c links with d then what happens if
suddenly b links with c?  

If you view linking as sharing a single backup account then you could
either throw an error, or else prompt to figure out if one or both of
the a or d go away. 

Alternatively, all wallets might posses their own independent backup
accounts, to which they can backup even before configured to do so, and
linking would merely be giving another wallet read and maybe notify
access.  It's slightly harder to protect metadata here, but not too bad
if the read access is symmetric, but that's similarly hard.

The design space keeps expanding...

Jeff

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