macro: add READ_NOW() macro for force reading of memory, making a copy

macro: add READ_NOW() macro for force reading of memory, making a copy

When accessing journal files we generally are fine when values change
beneath our feet, while we are looking at them, as long as they change
from something valid to zero. This is required since we nowadays
forcibly unallocate journal files on vacuuming, to ensure they are
actually released.

However, we need to make sure that the validity checks we enforce are
done on suitable copies of the fields in the file. Thus provide a macro
that forces a copy, and disallows the compiler from merging our copy
with the actually memory where it is from.