Calculate average transactions per second

Use OnTxAccepted notifications to track the time every transaction gets
accepted in the memory pool. With the use of a map, the average tps can be
estimated at the end of every simulation.

Plus:
* Removed ```a.WaitForShutdown``` for a deferred call to ```a.wg.Wait``` at start of ```a.Start```
* Close ```stop``` inside ```miner.go``` instead of just sending to it, since now with the addition of the goroutine calculating tps, two goroutines read from it.
* Removed sending from actor to ```a.stop``` since in a normal simulation the actor will block. Probably a separate channel should be used for sending back to main after an interrupt has occurred.
* Deferred```wg.Wait``` in ```Close```.
Please give this variable a more descriptive name.