Make Psr18Adapter not final and private members/methods protected

[thomascorthals]

I've turned Psr18Adapter into a non-final class and made all private members and methods protected instead. This is in line with the customisation philosophy described in the docs.

solarium/docs/customizing-solarium.md

Line 8 in 559f8a8

3. and also still supporting the extending of classes (Solarium doesn't use 'private' or 'final' anywhere)

This closes #795.

[codecov-commenter]

Codecov Report

Merging #796 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #796   +/-   ##
=========================================
  Coverage     90.61%   90.61%           
  Complexity     3530     3530           
=========================================
  Files           304      304           
  Lines          7627     7627           
=========================================
  Hits           6911     6911           
  Misses          716      716           

Flag	Coverage Δ	Complexity Δ
#unittests	90.61% <100.00%> (ø)	3530.00 <20.00> (ø)

Impacted Files	Coverage Δ	Complexity Δ
src/Core/Client/Adapter/TimeoutAwareTrait.php	50.00% <ø> (ø)	2.00 <0.00> (ø)
src/Core/Client/Adapter/Psr18Adapter.php	92.59% <100.00%> (ø)	23.00 <20.00> (ø)

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[mkalkbrenner]

I think we can close this now as the docs are adjusted.