Add skipLocked config for the database handler

[michalsn]

Description
This PR adds skipLocked config for the database handler.

Since using a "skip-locked rows" query can be problematic for some users, I decided to introduce a config option for this purpose.

If the developer runs only one instance of the same queue, it will be safe to set skipLocked to false and use it without worrying about the required database version. This is a fair compromise for those who do not use the queue intensively.

Needs: #47

Fixes: #42

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[coveralls]

Pull Request Test Coverage Report for Build 10935215757

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.433%

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Totals
Change from base Build 10935164404:	0.0%
Covered Lines:	487
Relevant Lines:	557

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💛 - Coveralls

[michalsn]

Thank you!