Make purchaseticket return immediately

[raedah]

When using purchaseticket, dcrctl does not return immediately because it is waiting for the join to complete on the server so it can return the txid. Instead, do not return the txid to the command line and return the dcrctl cmd immediately to the command line. The txid can be displayed in the dcrwallet log when it completes. This will also enable the user to be able to run the purchase ticket cmd multiple times.

[raedah]

This will be put on hold for now because of the complications it creates with utxo selection.