NSQLookupd support for nsq_consumer

[ljagiello]

This PR adds support for NSQLookupd in nsq_consumer.

Why:
Existing solution where nsq_consumer reads only from a single NSQD server limits ability to take advantages from distributed nature of NSQ.

NSQ godocs suggest to use nsqlookupd as a prefered method for connection (https://godoc.org/github.com/nsqio/go-nsq#Consumer.ConnectToNSQD):

It is recommended to use ConnectToNSQLookupd so that topics are discovered automatically. This method is useful when you want to connect to a single, local, instance.

nsqlookupd is the daemon that manages topology information. Clients query nsqlookupd to discover nsqd producers for a specific topic and nsqd nodes broadcasts topic and channel information.

With nsqLookupd support nsq_consumer will detect all avaiable nsqd producers and connects to all.

Additionally sync with the latest stable release of go-nsq (https://github.com/nsqio/go-nsq/blob/master/ChangeLog.md#107---2017-08-04)

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[ljagiello]

Tests failed on kafka_consumer_integration_test.go not related with this PR.

[danielnelson]

One concern with this change is that since it changes the type of the server field it will require users to update their config file. One way this could be avoided is by changing the new nsqlookupd boolean to be an array of nsqlookupd instances to connect to, but this method would not be very clear that you should choose either the server or nsqlookupd option and not both.

Looking over the nsq GoDoc, it looks like it is possible to both connect to a specific nsqd and at the same time be connected to nsqlookupd doing discovery, does it make any sense to combine these options? Perhaps someone would connect to a localhost instance while still being able to fallback to discovered instances?

Then we could have something like:

[[inputs.nsq]]
  ## Server option still works but is deprecated, we just prepend it to the nsqd array.
  # server = "localhost"
  nsqd = ["localhost"]
  nsqlookupd = ["localhost"]

and the plugin does both:

n.consumer.ConnectToNSQLookupds(n.NSQDLookupds)
n.consumer.ConnectToNSQDs(n.NSQDs)

[ljagiello]

@danielnelson sounds like a good idea, I will test that approach and provide necessary changes to PR.

[ljagiello]

@danielnelson now it's possible to use old setup or new setup or mix of 2.

[danielnelson]

Thank you! Will be included in 1.5.0.