WalletTool: use minBroadcastConnections == 1 when in regtest mode.
Immediately advance current addresses instead of doing it lazily. This avoids a bug whereby an app might quit after using a change address, thus currentKey(CHANGE) == null and it gets reset to the last used address when the wallet is round-tripped.

Unit tests didn't catch this because they didn't simulate the app terminating after the send, and weren't explicitly checking that the change address was different, so improve tests to do those things.

Additionally implement marking as used for married wallets.