Save multiple precache work values

[guilhermelawless]

Suggested by @zhyatt (thanks!).

Simply discarding work received by clients is wasteful. Even if we don't reward those, we can still compare their work value with the available one and replace if higher.

A more conscious approach is to save multiple work values, and when work is requested, use the one immediately above the current active difficulty. This creates a soft dependency on more node functionality. If broken, we can rely on the service to request what they think is an appropriate difficulty. We can also use always that mode, which puts the burden of tracking active difficulty on the services.