[#1465] Use pseudo-random generator for mk.number().

Problem description
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We want to use non predictable number generation for testing.

Changes description
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Initialize the number generation seed with a random value between 0, 5000.

How to try and test the changes
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reviewers @adiroiban 

Please let me know if you've changed your mind and would like to go for milliseconds at start. I've tested the change manually and run all the server tests with the new version. All tests pass.

Thanks!