header: trusting period should depend on network unbonding period

Currently, we hardcode the trusting period and the unbonding period. As the unbonding period is a network-wide parameter that lives on the state and can be adjustable, we cannot just hardcode and should check if it changes.

The trusting period should also be less than the unbonding period, and we may allow users to configure only the delta between unbonding and trusting instead of giving full control over trusted.