Fix Project Nodes multiple MQTT connections issue

[Steve-Mcl]

closes #62

Description

1. Adds a containment function to verify there is only ever 1 connection
2. unconditionally calls end with the force parameter set true after a normal end (regardless of success)

these 2 containment measures have proved to successfully maintain a single connection regardless of my attempts to break it.

Related Issue(s)

#62

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production

Labels

• Backport needed? -> add the backport label
• Includes a DB migration? -> add the area:migration label