Use memory purge redis command when initializing new kb

[ArnoStiefvater]

What:

Use memory purge redis command when initializing new kb.

Why:

Handle issue of redis memory not being released to the OS again.

How:

Running the following script with a target of 5 hosts with ospd-openvas. Redis will use about 800M memory.

if(description) {
  script_oid("1.3.6.1.4.1.25623.1.0.999999");
  script_version("2019-09-09T06:03:58+0000");
  script_name("test redis memory");
  script_category(ACT_SCANNER);
  script_family("Port scanners");
  script_copyright("This script is Copyright (C) 2009 Greenbone Networks GmbH");
  script_tag(name:"last_modification", value:"2019-09-09 06:03:58 +0000 (Mon, 09 Sep 2019)");
  script_tag(name:"creation_date", value:"2009-10-26 10:02:32 +0100 (Mon, 26 Oct 2009)");
  script_tag(name:"cvss_base", value:"0.0");
  script_tag(name:"cvss_base_vector", value:"AV:N/AC:L/Au:N/C:N/I:N/A:N");
  script_tag(name:"summary", value:"some value");
  exit(0);
}

include("misc_func.inc"); # for rand_str()

i = 0;
key  = "key";
value = rand_str(length: 1000);

a = rand();
b = rand();
for (; i<100000; i++ ) {
    set_kb_item(name: key+i+a+b, value:value+b+a);
}

Check the memory via e.g. redis-cli -s /run/redis-openvas/redis.sock info memory | grep used_memory_rss.
Run some other script which does not use much memory. The memory will probably not decrease to the baseline again.

With the PR the memory will imiidiately decrease to the baseline again if a script was started which does not use much memory after the initial script was called.

Checklist:

• Tests
• CHANGELOG Entry