Proto,Scoru: rename [size_diff] to [inbox_size_diff] to avoid confusion

Context

Closes #2342 (closed)

Originally tackling #2342 (closed) but we noticed that is already considered. We're then renaming the variable to avoid confusion.

Manually testing the MR

Checklist

• n/a Document the interface of any function added or modified (see the coding guidelines)
• n/a Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• n/a 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).
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