Improved code related to EZP-29139

[alongosz]

Question	Answer
JIRA issue	Follow-up on EZP-29139
Bug/Improvement	yes
New feature	no
Target version	6.x
BC breaks	no
Tests pass	yes
Doc needed	no

This PR introduces improvements to parts of the code related to #2423 changes.

TODO:

• Improve query selection logic in Content Gateway implementation
• Improve PhpDoc on the prototypes in the SPI URLAlias Handler.
• Fix new code according to Coding Standards ($ composer fix-cs).
• Ask for Code Review.

[alongosz]

Thanks 🎉
Merging, no QA needed as changed piece of code is rather well covered by integration tests.