Use primes for hashtable size

[Andrew Borodin]

Hi, nginxers!

We often use hashtable sizes equal to the power of 2. This can be
damaging for a hashtable. I haven't found any mitigation for this in
nginx code. So I made my own. If this issue is addressed somewhere
just ignore my message. Or I'd be happy if someone will point me it.

For the explanation of problem see
https://stackoverflow.com/questions/3980117/hash-table-why-size-should-be-prime

Code is checked for correctness of ngx_hash_min_prime(), I haven't
done any regression testing, sorry.

Also beware that I've changed state of paremeter hinit. I'm not sure
this is acceptable.

Thank you for your attention.

Best regards, Andrey Borodin, Octonica.
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