Move btcwallet kill, receive from spendAfter in a select

The reason behind ```btcwallet``` process kill being removed from ```a.Start``` is that it blocks the goroutine sending transactions. The process dies and the rpc call to ```SendFromMinConf``` never returns.

Also receiving from ```spendAfter``` is better to be done inside a select with ```a.quit```, since there is a possibility of a simulation ending before an actor acquires a mature coinbase.