Reduce invalid block age threshold

[jeffgrunewald]

Reduces the maximum block age a miner client will allow from a durable validator from 900 (roughly 15 min) to 600 (roughly 10 min) to ensure clients are ditching under-performing durable validators in favor of ones that are keeping better pace with the chain.

Also updates the timeout constant the client uses to trigger checks of a connected durable validator's block age to compute based on the maximum allowed block age. Keeps the formula previously in place but now computes it based on changes to the MAX_BLOCK_AGE to always be roughly half the allowed maximum block age in milliseconds. The check timeout is roughly half the allowed maximum block age plus a jitter of up to another half the allowed maximum block age.