Calculate average transactions per second

Set actors to send a list of their latest transactions back to main. Main with the use of a map can then calculate the average transactions per second.

Plus:
* Removed ```a.WaitForShutdown``` for a deferred call to ```a.wg.Wait``` at start of ```a.Start```
* Removed the send from one of the two actor goroutines, since, in a normally ending simulation, one of two would block.
* Moved ```wg.Wait``` from the end of ```Close```, in the middle just below the for loop where we ```a.Stop``` actors, since otherwise the ```btcwallet``` processes would die before their rpc clients returned from ```ListTransactionCount```.
Yup: https://github.com/conformal/btcrpcclient/blob/master/wallet.go#L155

Also, @jcrubino tried an alternate way by using a txn notification handler and collecting the notifications as they happened