[Epic] Refined contract interactions

[emizzle]

• Use "infinite allowance" for collateral deposits nim-codex#598
• maxCollateral should be linked to "per byte" instead of "per slot" nim-codex#602 More info: maxCollateralPerByte – instead of SP setting their maxCollateralPerByte, they would set their max storage size and max collateral. maxCollateralPerByte can then be calculated by storage available/collateral available. After each slot is serviced, the value would be recalculated for the next availability matching.
• Availability minPrice should be per byte nim-codex#460
• Reward address codex-contracts-eth#71
• Reward for validators that correctly mark proof as missing codex-contracts-eth#70
• Payout based on actual time filling the slot codex-contracts-eth#102
• Send client remaining funds when a contract fails codex-contracts-eth#98
• Content.cid should be bytes codex-contracts-eth#107
• Add proofProbability into pricing structure nim-codex#868
• Remove unneeded _probabilities? codex-contracts-eth#137
• Require proof for freeSlot codex-contracts-eth#143
• Maximum Request's duration codex-contracts-eth#101
• Replaceasserts with require codex-contracts-eth#149
• Return funds to client for failed request codex-contracts-eth#156
• Change all instances of host to provider in contracts
• Persistent availabilities nim-codex#467
• Remove missing proof leniency codex-contracts-eth#195

[AuHau]

@emizzle I have renamed the epic ("collateral" -> "contract") as while it tackles some of the collateral issues, it also tackles some general contract related issues ;-) Let me know if you think otherwise!