Scoru: relax dissection's distribution rule

Context

Fixes #3406 (closed)

Currently the node from

[
  (Some x, 0);
  (Some y, 62);
  (None, 63)
]

produces

[
  (Some x, 0);
  (Some _, 1);
  (Some _, 2);
  ...
  (Some _, 30);
  (Some y, 62);
] 

However, the last dissection has a length of 33, which is more than 62/2. Thus, it fails in the check dissection.

With the fix I propose for the rollup node, the dissection would rather be

[
  (Some x, 0);
  (Some _, 31);
  (Some _, 32);
  ....
  (Some _, 61);
  (Some y, 62);
]

Using a special jump on the first section length by using the remainder of trace lengt and number_of_sections, we have a distance for this section of 31. That is, we get in this bad scenario half + 1 distance at most.

However, half + 1 is still bad regarding the protocol check at the moment. I propose here also to relax this rule. To be honest, it does not change much if the maximum distance for a section is half of half + 1.

Manually testing the MR

I mirrored this implementation for the PBT tests in !5851 (merged). Using the randomness of these tests, we gain confidence on this dissection's building.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR