`FileCache`: better cleaning, free more than just the minimum

[albertz]

Freeing exactly need_at_least_free_space_size can be too little. E.g.:

• Multiple procs copy at the same time (e.g. RETURNN FileCache itself, see #1542, or other procs). After the space was freed, when the copying starts, or maybe during the copying, maybe some of the space was already taken again.
• We actually need one more byte due to the current logic in _copy_with_prealloc.
• I'm not sure if the filesystem maybe needs a few more bytes, e.g. for creating directories or so.

Another problem is that the current cleanup_files_wanted_older_than_days default (7 days) is maybe way too high and is rarely used when it matters (to free enough space). Although, maybe some of the other suggestions below already are enough. Not sure.

Some suggestions:

• Fix need_at_least_free_space_size = os.stat(src_filename).st_size + 1.
• Always try to free more than need_at_least_free_space_size, maybe sth like 2 * need_at_least_free_space_size or so. However, if this fails, it's only a warning. Only if we fail to free need_at_least_free_space_size, it would be an error.

Other suggestions?