Add more Stream and Consumer management APIs

[variadico]

This adds the follow APIs for Streams.

• UpdateStream
• DeleteStream
• SnapshotStream
• RestoreStream
• PurgeStream
• StreamLister

This adds the follow APIs for Consumers.

• ConsumerInfo
• ConsumerLister
• DeleteConsumer

[coveralls]

Coverage decreased (-0.8%) to 86.901% when pulling 3e0aed6 on add-management into b4450fb on master.

[derekcollison]

Be good if we could hide the paging at this layer or undo it below the covers and just make it a stream of messages with EOF.

[variadico]

All right. I added a StreamLister and ConsumerLister to iterate through pages.

This is what the calling code looks like.

sl := js.NewStreamLister()
for sl.Next() {
	// sl.Page() returns a []*StreamInfo.
	// Each iteration can return a maximum of 256 items.
	fmt.Println(sl.Page())
}
if err := sl.Err(); err != nil {
	return err
}

Here's what it looks like in tests.
https://github.com/nats-io/nats.go/pull/626/files#diff-9e6bdee4051a6e36d6d507dcef63b5f9a39b281e421c08f02ec74a0fb6639e3aR592-R626

[kozlovic]

Some little changes needed I believe.

[variadico]

Okay, @kozlovic. I think I resolved all of the comments you had. Thanks for the review. 👍

[kozlovic]

I would be ok with the change, but I have asked other to chime in with regards to the addition of the timeout in the stream snapshot config struct.

[kozlovic]

Check snapshot config for nil and deliver subject.

[kozlovic]

LGTM

[kozlovic]

@variadico Not sure why you removed snapshot? I think @ripienaar comment was about Timeout in config, but I may be wrong. Clearly, we can always add features later, so I don't have a problem removing, just curious as to why you remove it.

[variadico]

@wallyqs wanted some more time to think about the snapshotting logic, so I removed that from this PR. That'll be added later. Additionally, he's going to add a few more management APIs.

[derekcollison]

We should have DeleteMsg and EraseMsg for the Stream Management API..

[wallyqs]

LGTM, also adding DeleteMsg as a different PR here: Adding DeleteMsg as a separate PR here #631