stable certificates part2

stable certificates part2

Two different certificates of the same fake Issuer must have the same serial
number. Otherwise, Firefox and possibly others will display a
sec_error_reused_issuer_and_serial error. Similarly, the same two certificates
should have the same serial number, even if generated on different
non-communicating (but identically configured) Squid boxes.

To produce unique serial numbers a temporary fake certificate with serial number
zero created, and its fingerprint used as the serial number of the final fake
certificate.

The old Ssl::CertificateDb code which was responsible to produce a serial number
for generated certificates removed.