Implement barrier for phases

[marcomicera]

Right now, the master waits for all the modules of a given phase using a counter and go channels:

broker-test/master/master.go

Lines 123 to 134 in dbc4cbb

	// Waiting for modules' responses.
	// Apparently, we cannot wait for multiple messages belonging to different queues.
	// However, it makes sense for Quartermaster to wait only for messages belonging
	// to the current phase (i.e., builder, analysis, report).
	// The master consumes messages following that order.
	allModulesHaveResponded := make(chan bool)
	remainingModules := len(modulesToWakeUp)
	consumeModuleResponse(ch, &builderResponseQueue, allModulesHaveResponded, &remainingModules)
	consumeModuleResponse(ch, &analyzerResponseQueue, allModulesHaveResponded, &remainingModules)
	consumeModuleResponse(ch, &reporterResponseQueue, allModulesHaveResponded, &remainingModules)
	<-allModulesHaveResponded

The counter is being decremented in the module response callback:

broker-test/master/master.go

Lines 69 to 72 in dbc4cbb

	*remainingModules -= 1
	if *remainingModules == 0 {
	finalModuleHasResponded <- true
	}

Is there a better way to handle this?