Bugfix: change time when loaders are cleared

[paullatzelsperger]

What this PR changes/adds

To avoid inconsistent loader storages, the time, when they are cleared was changed.
Now, all loaders are cleared just before the ExecutionPlan is scheduled.

Why it does that

Do avoid inconsistent loader storages.

Further notes

• the pre-executionHook could be used for other tasks as well.
• theoretically we could add a post-executionHook as well, but that was not needed as of now.

Linked Issue(s)

Closes #1495

Checklist

• added appropriate tests?
• performed checkstyle check locally?
• added/updated copyright headers?
• documented public classes/methods?
• added/updated relevant documentation?
• added relevant details to the changelog? (skip with label no-changelog)
• formatted title correctly? (take a look at the CONTRIBUTING and styleguide for details)