only set a new random seed if _the last_ Generator gets destruct'ed

[themasch]

What is the reason for this PR?

In cases where multiple Generator instances might be created, a gc cycle collection run can trigger at any time, potentially removing "old" Generator instances, which then calls __destruct and thus seed, which reset the global mt_rand seed, impacting everyone else.

If another, still live instance of Generator gets used after that, its output is no longer deterministic, since a new random seed was set.

• [ ] A new feature
• [x] Fixed an issue (resolve #870 )

Author's checklist

• [x] Follow the Contribution Guide
• [ ] New features and changes are documented

Summary of changes

I added a new static class property to Faker\Generator which counts the number of currently existing Generator instances: It gets incremented in construct, and decremented in destruct. Only if this counter reaches 0 does __destruct also call seed to restore the system to a "properly random" state.

Using a global counter is, imho, quite ugly, but since Generator::seed already modifies global state anyway I think its like fighting fire with fire. Other than that, I think its a quite minimal change that does not impact any other use of Faker. Cases that "properly" only ever use exactly one instance of the Generator will not see any difference in behaviour.

It's not a perfect solution by any means. But I think it is a reasonable one.

Review checklist

• [ ] All checks have passed
• [ ] Changes are added to the CHANGELOG.md
• [ ] Changes are approved by maintainer

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