Wrong shielded balance in query

[grarco]

If the shielded context is in a speculative state, the call to query the shielded balances will most probably return a wrong value.

A first workaround could be to print a line in the client to warn the user about this and ask them to fetch again to see the updated balances.

A better approach would be to address #2593 and update the shielded context only if the transaction is successful: in this case the response we'd get from the query would be correct.

[cwgoes]

Let's make sure to note this w.r.t. shielded sync @batconjurer @sug0