Add communication between actors and main

Multiple actors will have to communicate between them by sending
transactions to each other so a prototype of this communication is
created here where the main goroutine plays the part of a coordinator.
A OnBlockConnected notification has been added to notify main to stop
transcations when the someBigNumber-th block, after the actors have
started running, has been added in the blockchain.

Other than that, almost every use of log.Fatalf has been dropped to
log.Printf, use of a time-dependant random seed has been added in favor
of purely random address selection inside actors and method BalanceUpdate
has been added for Actors.
Probably better to declare that magic number as a constant (scoped to this function is fine if it's not needed anywhere else).