zcash_client_backend: Report confirmations of pending balance in wallet summary.

[nuttycom]

Return the minimum over the number of confirmations for the set of notes that are included in the pending balance.

This can be used to make rebroadcast determinations, and can also be used to provide an estimate of time until balance is spendable if wallets want to be able to display that.

[str4d]

The idea here is to give wallets an indication of (for the current wallet state) the maximum chain time they need to wait until the currently-reported balance becomes spendable.

This will interact with ZIP 315, and probably shouldn't be directly presented as "number of confirmations" as that requires the callers to know the end point. Instead, we should report something like "number of confirmations that still need to occur", which could equivalently be represented by the block height at which that number becomes 0.