spend: include only those unconfirmed coins from self for coin selection

[jp1ac4]

In #873, we included unconfirmed change for coin selection when creating a new spend. The intention of only considering change was to include only those unconfirmed coins that were not dependent on external transactions, but a coin being at a change address does not guarantee this.

Following #1483, we can make this condition exact by only including those unconfirmed coins that are from self.