Set default fallback duration to `MAX`.

[afck]

Motivation

The validators are not currently expected to start client processes automatically. So chains that enter fallback mode are effectively bricked.

Proposal

Set the default fallback timeout to MAX, so validators will never sign a fallback timeout certificate.

Test Plan

(Only a default chain config change.)

Release Plan

• Nothing to do / These changes follow the usual release cycle.

Links

• reviewer checklist