[SERVER] Use O_DIRECT for integrity checks

The idea is that for full image checks, we don't want to
pollute the fs cache with gigabytes of data that won't be
needed again soon. This would certainly hurt performance
on servers that dont have hundreds of GBs of RAM.
For single block checks during replication this has the
advantage that we don't check the block in memory before
it hit the disk once, but actually flush the data to disk,
then remove it from the page cache, and only then read it
again, from disk.

TODO: Might be worth making this a config option

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