Feature: Pre-registered links and simplified client-server setup

[kelindar]

Overview

This PR introduces a new concept of links which brings a couple of benefits.

1. It helps to cope with the short message length and the limited transmission bandwidth in wireless networks, the pre-registered channel name in the PUBLISH messages can replaced by a short, two-byte long "link name". A registration procedure is defined to allow clients to register their channel names with emitter broker by sending a request to emitter/link/ channel.
2. It simplifies the creation of trust relationship between a client and a server by (a) auto-creating a sub-channel with suffixed connection identifier and (b) optionally auto-subscribing to that private link. A secure channel key is transparently created by the broker for the auto-created channel and the key is used when publishing to the link.

Creating Links

In order to create a link, a JSON-formatted message must be sent to emitter/link/ MQTT topic. The message should contain at least the alphanumeric name (max 2 characters), a valid key and a channel. It optionally can contain a "subscribe" or "private" flags.

If subscribe flag is set to true and a channel key has a subscribe (read) permission, then emitter broker will automatically subscribe the client to the channel.

{ 
   "name": "0", 
   "key": "k44Ss59ZSxg6Zyz39kLwN-2t5AETnGpm", 
   "channel": "a/b/c/", 
   "private": false, 
   "subscribe": false
}

If the private flag is set to true and a channel key has an "extend" permission as shown in the screenshot below, then the channel will be made private by appending a connection identifier string to the provided channel.

{ 
   "name": "1", 
   "key": "k44Ss59ZSxg6Zyz39kLwN-2t5AETnGpm", 
   "channel": "a/b/c/", 
   "private": true, 
   "subscribe": true
}

For example, if you request the above private link to be created, 3 things will happen on emitter broker:

1. The broker will create a sub-channel with a secure key for the new private link (e.g. .../a/b/c/F45JPXDSXVRWBUKTDNCCM4PGQ/ )
2. The broker will subscribe (as requested) to that private sub-channel.
3. The broker will map a link with name 1 to that private sub-channel (e.g. 1 -> .../a/b/c/F45JPXDSXVRWBUKTDNCCM4PGQ/

Then, by simply issuing a PUBLISH onto the topic 1 the broker will route the message in the private channel. Note that you can still manually create a wildcard key for a/#/ in order to create a "server" client which will be able to see everything.

[coveralls]

Coverage decreased (-0.2%) to 90.722% when pulling 829fe3c on feat-dial into 656216c on master.

[Florimond]

LGTM