[WIP] Synchronous and Asynchronous reconnection

[fenos]

Reconnection Proposal

When NATS or others MessageBrokers loses connection permanently we are unable to re-establish a connection.

This PR will allow to you to opt in for the abstract ReconnectorSink which will handle re-connection logic for your MessageBroker.

The implementation allows you to chose between Asynchronous or Synchronous reconnection logic.

• Asynchronous (Default): When losing some messages it's not a big deal
• Synchronous: When loosing messages it's a big deal!

Example Usage:

	sink, err := instrumented.NewReconnectorSink(
		natss.ErrConnLost, // The error which identify a Connection Lost
		func() (pubsub.MessageSink, error) { // Function that returns your interested Sink
			// You can return any SINK you'd like
			return natss.NewMessageSink(
				natss.MessageSinkConfig{
					NatsURL:   url,
					ClusterID: clusterID,
					ClientID:  clientID,
					Topic:     topic,
				},
			)
		},
		&instrumented.ReconnectionOptions{
			ReconnectSynchronously: false,
			OnReconnectFailed: func(error) {
				// Logging
			},
			OnReconnectSuccess: func(sink pubsub.MessageSink, err error) {
				// Do Somenthing
			},
		},
	)

connects to utilitywarehouse/crm/issues/152

[houndci-bot]

if block ends with a return statement, so drop this else and outdent its block

[houndci-bot]

if block ends with a return statement, so drop this else and outdent its block

[denislobanov]

I think this could be really useful, but I can't help but wonder why is Async default? Given the explanation that async allows for message loss; would it not be more sensible to set default to synchronous?