Previously there was a tx cache that you passed in as part of the sync
request. This seems bad and the example show'd that you should copy all
your transactions from the transaction graph into the sync request every
time you sync'd. If you forgot to do this then you would always download everything.
Instead just do a plain old simple cache inside the electrum client.
This way at least you only download transactions once. You can
pre-populate the cache with a method also and I did this in the examples.
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