Add support scanning and sending transparent transactions for MemoryWallet

[willemolding]

Transparent txns and balances are handled in a completely different way to shielded ones. Since transparent txns can be scanned by lightwalletd without compromising quality this is exactly how it is done. This is much more similar to a conventional light-wallet.

Completing this task requires implementing:

• store_transactions_to_be_sent update to support transparent
• get_transparent_balance implemented
• get_transparent_receivers implemented

See https://github.com/ChainSafe/zec-sqlite-cli/blob/8c2e49f6d3067ec6cc85248488915278c3cb1c5a/src/commands/sync.rs#L602 for further details on how lightwalletd is used when scanning transparent balances