refactor: replace `that` with `this` by using arrow functions

[JKRhb]

This PR removes the

const that = this

statements from the codebase and replaces occurences of that with this by using arrow functions which allow for calling this directly. This should make the code easier to read.

[coveralls]

Pull Request Test Coverage Report for Build 1273023552

• 67 of 75 (89.33%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.06%) to 89.814%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/server.js	15	18	83.33%
lib/agent.js	35	40	87.5%
<!--	Total:	67	75	89.33%	-->

Totals
Change from base Build 1272738547:	-0.06%
Covered Lines:	1079
Relevant Lines:	1170

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💛 - Coveralls

[Apollon77]

thank you :-)) great improvement