Commission decrease should be allowed at any time

[bji]

Problem

The commission_updates_only_allowed_in_first_half_of_epoch feature disallows commission decrease in the second half of epoch. Given that the purpose of this feature was to prevent 'rug pulls' which are accomplished by increasing commission at the end of epoch and then decreasing commission at the beginning of next epoch, disallowing decreases during the second half of epoch is unnecessary.

Disallowing commission decreases in the second half of epoch prevents an operational technique that I use with my validator: when I have an operator error that results in reduced stake account rewards for the epoch, I reduce commission to 0% to ensure that stake accounts are not disadvantaged by my actions. Not being allowed to do this in the second half of an epoch is a real problem for me.

Proposed Solution

Add a feature which will enable set-commission logic to first check whether the proposed commission change is a decrease, and if so, do not invoke the "only allow commission change in first half of epoch" rule.

[joncinque]

The feature gate issue to track this is #33994