Implement signaling on protocol methods replacing sleeping gorutines.

[CPerezz]

Derived from this discussion of #141:

I am not convinced by your explanation. Either the process is synchronous (I send a findNodes request and I wait for some kind of side effect, like a response that may or may not be related to the message I broadcasted) and therefore the process should block until some condition is met (whatever the condition). In case the process is instead asynchronous, sendFindNodes should follow a fire-and-forget model (and therefore there should be no followup in the same thread/goroutine basically). In both cases, the sleep is inappropriate.

I think that what you want might be an heuristic condition, best modeled with a sync.WaitGroup

Originally posted by @autholykos in #141

We decided that using sync.WaitGroup might be a better solution for the steps done in protocol.go -> Bootstrapping and Network_Discovery.