Improve structure

[Kraemii]

What:
Improving the overall structure of ospd-openvas.
Currently changed the following:

• Scan-loop structure
• Zombie-process handling
• Stopping a scan

Depends on: greenbone/ospd#432

Why:
There are many bugs and interrupted scan issues which can't be reproduced. On top of that the ospd-openvas code is hardly readable and need some improvements to make it easier to hunt bugs and work on issues.

How:
Just testing some scan configs and the stop scan feature to make sure everything is working as expected.

TODO:

• Adjust Unittests

Checklist:

• Tests
• CHANGELOG Entry

[codecov]

Codecov Report

Merging #450 (cf46955) into master (4489242) will decrease coverage by 0.00%.
The diff coverage is 24.00%.

@@            Coverage Diff             @@
##           master     #450      +/-   ##
==========================================
- Coverage   80.30%   80.30%   -0.01%     
==========================================
  Files          10       10              
  Lines        1681     1665      -16     
==========================================
- Hits         1350     1337      -13     
+ Misses        331      328       -3     

Impacted Files	Coverage Δ
ospd_openvas/db.py	97.05% <0.00%> (ø)
ospd_openvas/daemon.py	65.07% <17.77%> (-0.55%)	⬇️
ospd_openvas/openvas.py	100.00% <100.00%> (ø)

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