Limit mixed autobuyer ticket purchases to one per cxn
Limit mixed autobuyer ticket purchases to one per cxn

This results in up to --ticketbuyer.limit number of separate
connections to the CoinShuffle++ server triggered by each connected
block, instead of a single connection that funds several ticket
purchases but only creates a single change output.

This will result in slower mixed ticketbuying if the funding account
has limited UTXOs, as separate requests means that each individual
ticket purchase must be funded by a separate set of UTXOs.

decred/dcrwallet,decred/dcrwallet