zcash_client_sqlite: Identify shielding transactions in the output of the v_transactions view.

[nuttycom]

The specific ask here is to identify transactions that were created as a result of clicking the "shield funds" button in a wallet (i.e. by calling propose_shielding). These transactions have the following characteristics:

• Only transparent inputs.
• Only shielded outputs.
• Inputs are all from the wallet.
• Outputs are all to the wallet.
  • i.e. this is a wallet-internal transaction, though for historic wallets the outputs might not have been sent to wallet-internal addresses.

[str4d]

We can't implement exactly the above in v_transactions:

• We do not record any details about other inputs beyond what we ourselves have spent.
• We do record details about other outputs in the sent_notes table, but probably not as reliably as we'd need.

However, we can implement the following check:

• Only transparent inputs from our wallet.
• Only shielded outputs to our wallet.

This can be done by adding a count of does_not_match_shielding that we increment when we observe a spent note that is not from the transparent pool, or a received note that is to the transparent pool. Then sum that counter and is_shielding is when the counter is zero. This would also catch transactions that might be shielding but also potentially have other inputs and outputs; these could be eliminated by fetching the raw transaction to confirm directly.