Core: Generate realistic bounds in benchmarks

[aokolnychyi]

This PR makes FileGenerationUtil, which is used in benchmarks, generate realistic bounds.

Our current logic always generates 16 random bytes, which causes problems:

• The metrics mode is not respected (none, count, full).
• Bounds may be invalid values (int/long).
• Lower bounds may be actually be greater than upper bounds.

 byte[] lower = new byte[16];
 random().nextBytes(lower);
 lowerBounds.put(fieldId, ByteBuffer.wrap(lower));
 byte[] upper = new byte[16];
 random().nextBytes(upper);
 upperBounds.put(fieldId, ByteBuffer.wrap(upper));

I ran a few benchmarks that depend on FileGenerationUtil after this change and they seem to run fine. I also added tests.

[singhpk234]

LGTM, Thanks @aokolnychyi !

[aokolnychyi]

Thanks, @singhpk234 @danielcweeks @Fokko @amogh-jahagirdar!