client/rpc: Add abandontx.

[JoeGruffins]

I have had a few instances on mainnet where the dcr wallet thinks I have a tx that never happened. It most recently happened with a ticket that did not exist, but it continued to try and confirm it in order to pay the fee. Rescans did not fix the situation. There is definitely a better answer for these specific problems, but in the short term for times like these, and possibly as part of a resolution to #3135 it would be good to have a way to abandon transactions. An rpc enpoint would be a start.