Shredding heavy files can cause memory overflow

[sundowndev]

Previously, we used to write random data to files by set of 1024 bytes. Which means that for a file of 4096 bytes, we perform 4 write operations to rewrite the file entirely. Since https://github.com/sundowndev/covermyass/pull/10, we're now doing a single write operation per file. This can lead the program to crash if the file is too heavy (we should perform some benchmarks to confirm that). Doing a progressive shredding instead can solve that case.