Flush symbolic links and filesystem entries

[rbs-jacob]

• [x] I have reviewed the OFRAK contributor guide and attest that this pull request is in accordance with it.

One sentence summary of this PR (This should go in the CHANGELOG!)

Better flushing of files to disk.

Link to Related Issue(s)

362

Please describe the changes in your request.

• Add FilesystemEntry.flush_to_disk (previously only in FilesystemRoot)
• Refactor FilesystemRoot.flush_to_disk to use FilesystemEntry.flush_to_disk
• Rename Resource.flush_to_disk to the more accurate Resource.flush_data_to_didsk to indicate that it only flushes data, not any attributes

Anyone you think should look at this, specifically?

@EdwardLarson