Separate every functional change in yunikorn-core fork to decrease chance of merge conflicts

[nikola-jokic]

When we sync fork in the future, we want to avoid having merge conflicts as much as we can. Since the intention is not to modify the scheduler behavior, the easiest way to decrease chance of merge conflicts is to move functionalities that we implemented into separate files.

Some changes will have to still exist inside the files forked from yunikorn-core. For example, fields added to the struct will still have to exist in files that will be synchronized with the upstream.

However, we can extract functionalities like snapshots and unlocked creation of dao objects by moving them to a separate file.