Cleanup

After the updated utxo selection process, some of the old code related to coinbase is no longer required.

Also re-used `safeClose` and fixed hang when `a.utxo` receives and blocks.
Not sure on this part. You probably don't want to start another drainer and ask the rpc client to shutdown again (although I guess it can't hurt, but it's just silly, and silly code usually ends up being the first to break in the future).