`ElectrumResultExt` trait is also introduced that adds methods which can
convert the `Anchor` type for the update `TxGraph`.
We also make use of the new `TxCache` fields in
`SyncRequest`/`FullScanRequest`. This way, we can avoid re-fetching full
transactions from Electrum if not needed.
Examples and tests are updated to use the new `ElectrumExt` API.