Improve comments and do some renaming and restructuring

[gbalabasquer]

PR which includes:

• Improved comments
• Some renaming
• Some restructuring
• Overflow validation in simple market last offer id
• Same than: https://github.com/makerdao/maker-otc/pull/184

[gbalabasquer]

We will keep it simpler making much less changes to the code. This PR will be closed for now and we will just focus on #181.