Add option to avoid spending unconfirmed change

Adds the option `-spendzeroconfchange=0` to make the wallet avoid spending zero conf change outputs.

Rough workaround to the problems with '-4. Error: The transaction was rejected!' and other problems with chains of unconfirmed transactions and malleability.

The drawback of this is that sending will run out of inputs sooner (resulting in unintuitive "insufficient balance" errors), so if you have a lot of sends you'll need to queue up sends and use sendmany.