Save list of incoming transactions for current address during snapshot

[joshirio]

Currently the history of an address is saved to the wallet file permanently only after an outgoing transaction, which implies that the address will never be used again, since that would trigger an address reuse.

Currently, if a snapshot happens, an unspent address will lose its incoming transactions history, since nodes drop all transactions after a snapshot.

It makes sense to always store the latest transaction in the wallet file under a temporary position that is added to the wallet file permanently if a snapshot has been detected.

It is necessary to find a reliable way to detect a snapshot, because sometimes broken nodes report zero transactions for a given address.

[joshirio]

Issue: after current snapshot some CarrIOTA nodes that haven't upgraded show old transactions while other upgraded nodes return no transaction, this means checking for an empty tx list to detect snapshots is not reliable because the next node could be showing the old list again causing multiple entries for the same transactions.

Maybe there's another way. A possibility would be to save every new incoming transaction based on bundle hash to detect unique transactions and to avoid duplicates.