Fix "close timed out" error when performing full deploy or modifying broker node.

[Steve-Mcl]

fixes #2934

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

Prevent the close timed out issue (see references below)

• Move all calls to node.client.end() to one location in code base (broker node function .disconnect() ) to reduce code paths and early calls.
• Ensure done is always called for the "close" event in MQTT In/Out/Broker nodes (node.on('close') by removing the done callback from the node.client.end() signature - since it is (at best) unreliable due to the myriad of combination of user options and order of events (i.e. which nodes close first, flow file ordering, whether node has a close packet to sent etc)

References...

#2934
martin-doyle/node-red-contrib-aedes#46
martin-doyle/node-red-contrib-aedes#54
https://discourse.nodered.org/t/error-stopping-node-close-timed-out-mqtt-nodes/52830/5
https://discourse.nodered.org/t/mqtt-close-timeout-issue-using-aedes/57583
https://discourse.nodered.org/t/mqtt-in-node-not-working-properly-in-node-red-2-2-1/58428
https://discourse.nodered.org/t/mqtt-connects-then-disconnects/39609/6

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage increased (+0.002%) to 67.321% when pulling 669aa76 on Steve-Mcl:fix-mqtt-close-timeout into fcf2994 on node-red:master.