Often when the wallet creates a tx, the flow is:
- create unsigned tx
- sign tx
- broadcast tx, but don't save it in history
- server sends notification that status of a subscribed address changed
- client calls scripthash.get_history
- client sees txid in scripthash.get_history response
- client calls blockchain.transaction.get to request missing tx
Instead, now when we broadcast a tx on an interface, we cache that tx *for that interface*,
and just before calling blockchain.transaction.get, we lookup in the cache.
Hence this will often save a network request.
