refactor(microservices): update server properties to protected

[barlock]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

TCP server and mqtt client were private so subclasses couldn't access them.

I peered through a few of the other servers and clients, this seems to follow patterns I've seen.

What is the new behavior?

downstream projects can subclass and access the server and client directly! YEY!.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[coveralls]

Pull Request Test Coverage Report for Build c777d6b3-63c5-46f4-9d8d-3aadaf033241

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.786%

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Totals
Change from base Build 6b637f64-9c7a-4d7b-8537-b6a8d098f58e:	0.0%
Covered Lines:	6113
Relevant Lines:	6518

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💛 - Coveralls

[kamilmysliwiec]

lgtm