BUG/MINOR: ssl: Be sure to use unique serial for regenerated certificates

BUG/MINOR: ssl: Be sure to use unique serial for regenerated certificates

The serial number for a generated certificate was computed using the requested
servername, without any variable/random part. It is not a problem from the
moment it is not regenerated.

But if the cache is disabled or when the certificate is evicted from the cache,
we may need to regenerate it. It is important to not reuse the same serial
number for the new certificate. Else clients (especially browsers) trigger a
warning because 2 certificates issued by the same CA have the same serial
number.

So now, the serial is a static variable initialized with now_ms (internal date
in milliseconds) and incremented at each new certificate generation.

(Ref MPS-2031)