Some PluggableMonitor client improvements

[cmaglie]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?
  This PR adds a bunch of improvements to the pluggable monitor client:

1. Added QUIT command support
2. Use rpc.Port in Open method (as it should be)
3. Slighlty improved error messages
4. Applied the same fix we did on PluggableDiscovery client about resource leaking

This PR is preparatory for the bigger Pluggable Monitor implementation.

• Does this PR introduce a breaking change, and is
  titled accordingly?
  No

[silvanocerza]

Add the [skip changelog] before merging please. 🙏