Wallet: Add option `lock` to tx creation methods.

If transaction is not broadcast, coins wont get locked. Which can lead to the situation where next tx creation may also reuse those coins. Ability to lock used coins before they are inserted into txdb is useful for automation using external signing tools. (e.g. ledger)

#845

Start with the wallet transaction creation methods and the related issue #845, then trace how selected coins are handled before insertion into txdb. Done means an optional lock setting can lock used coins before txdb insertion when transactions are not broadcast, with coverage for external-signing automation.

Written by the indexing model from the issue text.