Further refine referral system terminology

[lightwalker-eth]

In #1874 we made a number of improvements, including renaming some fields to uncappedAward and cappedAward. This is good terminology! I note that we have a misalignment remaining from this: We're using the word "limit" many places in rev-share-limit. Each instance of the word "limit" should instead be "cap". For example, rev-share-limit should be renamed to rev-share-cap for consistency.

This terminology update should be applied across all relevant code and file names.