`DefaultPendingChildKeyCooldown` should use `prod_or_fast!(7_200, 15)` instead of `0`

[basfroman]

Context:

1. function set_children executes do_schedule_children
2. function do_schedule_children uses PendingChildKeyCooldown storage to get cooldown_block
3. storage PendingChildKeyCooldown uses DefaultPendingChildKeyCooldown which is 0 by default.
   Theoretically, if we compile a chain from scratch without history, we'll get PendingChildKeyCooldown equal to 0 instead of 7200/15. This happens with localnet now.

Fix should include:

1. DefaultPendingCooldown is prod_or_fast!(7_200, 15)
2. DefaultPendingChildKeyCooldown gets from DefaultPendingCooldown.