Convert "severity_date" value to seconds since epoch

[jjnicola]

What:
The tag names "severity_date" is treated special

Why:
For consistency, the severity_date date format value is the same as for creation_date and last_modification. Internally is expected to be a seconds since epoch value.

How:
Add the following tags to a script

  script_tag(name:"severity_vector", value:"CVSS:3.0/AV:L/AC:H/PR:H/UI:R/S:U/C:N/I:L/A:L");
  script_tag(name:"severity_origin", value:"greenbone");
  script_tag(name:"severity_date", value:"2009-03-26 19:23:59 +0100 (Thu, 26 Mar 2009)");

Check in redis cache, that the severity date for the modified script is seconds since epoch.

redis /tmp/redis.sock[1]> lrange nvt:1.3.6.1.4.1.25623.1.0.100081 0 -1
 1) "auth_enabled.nasl"
 2) ""
 3) ""
 4) ""
 5) ""
 6) "Services/auth, 113"
 7) "find_service.nasl"
 8) "last_modification=1549632112|creation_date=1238091839|cvss_base_vector=AV:N/AC:L/Au:N/C:P/I:N/A:N|severity_vector=CVSS:3.0/AV:L/AC:H/PR:H/UI:R/S:U/C:N/I:L/A:L|severity_origin=greenbone|severity_date=1238091839|summary=The remote host is running an ident daemon.\n\n  The Ident Protocol is designed to work as a server daemon, on a user's\n  comp

Checklist:

• Tests
• CHANGELOG Entry

[mattmundell]

Looks fine except for the little formatting issue.