Add a ProcessAssertion cache to avoid hammering RunningBoard when switching quickly from one assertion type to another (e.g. background <-> foreground). Taking RunningBoard assertions is expensive and this churn can result in high CPU usage. We have evidence this can happen for example when we're holding a background assertion on the WebProcess but the client app keeps running JS in the view, causing us to take a foreground assertion for each JS execution request. We now keep previous assertions around for 3 seconds when switching from one assertion type to another and we reuse them when possible.