`updateWeightedAverage` safety concerns

[jalextowle]

Goal

updateWeightedAverage should satisfy the invariant that _average >= min(oldAverage, delta) && _average <= max(oldAverage, delta) in Hyperdrive deployments.

Deliverables

• Unit tests for updateWeightedAverage that identify the dangerous regimes. A matrix based test is the stretch goal.
• Evaluate how updateWeightedAverage is being used. Which dangerous regimes are we worried about? Include documentation inline that describes which regimes we are avoiding and how we are avoiding them.
• Configuration testing. For DAI, ETH, and USDC, we should have tests that verify that updateWeightedAverage will never violate its invariant in normal operation.
• Maybe: If the current implementation can't pass configuration testing (e.g. there are possible inputs that can break updateWeightedAverage), then we'll need a fix. This probably looks something like a minimum bond amount.