[server] Use the `multiprocess` library on every platform

[vodorok]

Multiprocessing is broken on osx, (probably because of the fork -> spawn change). This patch intends to correct it by using the newly added multiprocess library.

[vodorok]

Did some tests on windows, and I could use the multiprocess lib without issues. I will remove the branching completely.

[whisperity]

Did some tests on windows, and I could use the multiprocess lib without issues. I will remove the branching completely.

@vodorok Let's just make sure these claims hold true for sufficiently old Python versions such as 3.9. Other than that, I think this is good to go.

[vodorok]

Do not merge yet Edit: more explanation: The web tests got really flaky after non functIon change modification (changed import order). I am not completely sure why is this happens, but there can be cases where the zip file disappears during the store process.

[whisperity]

1. Create a new file codechecker_common/multiprocessing.py which deals with the platform-specific branching.

if sys.platform in ["darwin", "win32"]:
  from multiprocessing import Pool as MultiProcessPool
elif sys.platform == ":
  from concurrent.futures import ProcessPoolExecutor as MultiProcessPool

2. In the usage places, import this new module and use the aliased name

from codechecker_common.multiprocessing import MultiProcessPool

# ...

with MultiProcessPool(...) as pool:
  ...(..., pool.map)

[vodorok]

Made a different implementation where the linux platform still uses the ProcessPoolExecutor class. See #4118 Will debug this after the release.