ENH: HDFStore.flush() to optionally perform fsync (GH5364)
Pull request for #5364.

The `fsync=False` parameter was added to `flush` given this offers utility for non-distribution use cases, such as a user just wanting to ensure their data is regularly flushed to disk.

I could not see any mocking library in use in the Pandas tests, so I skipped adding one just to verify `os.fsync` actually gets called.

For now, probably not worth it, because we don't have much that really needs it (might be nice for network tests but a little too late now).
 thanks for this!
