Use ScanSummary to reduce number of wallet balance updates

[str4d]

The ScanSummary data exposed by Electric-Coin-Company/zcash-light-client-ffi#113 includes the number of notes that were detected as spent or received due to scanning the range. We can use this to reduce the number of getWalletSummary FFI calls; we don't need to update the Sapling balance if no Sapling notes were spent or received.