make server variable public

[menta2k]

The use case:
I have have a multiple instance of atreugo in single application. They have kind of dependency between each other.
I like to shutdown both instances gracefully using context with timeout in certain order. The only way to achieve this is to implement a custom graceful stop. To do that i need a direct access to fasthttp instance in atreugo structure or expose fasthttp shutdown function

[coveralls]

Pull Request Test Coverage Report for Build 473

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 470:	0.0%
Covered Lines:	709
Relevant Lines:	709

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💛 - Coveralls

[savsgio]

And fix the linter error, please.

[savsgio]

If you need the shutdown function only, i prefer to keep non public the internal fasthttp server instance. So please, remove it.

[savsgio]

@Menta2L