sch-UID2-5352 honouring refresh from in salt rotation

[sophia-chen-ttd]

Salts will only be considered for rotation if their refresh_from is equal to the target date. Feature is behind a feature switch, which can be enabled through the admin config file.

Unit test:

• salts that are old enough to rotate and are refreshing on target date will be considered for rotation
• salts that not old enough to rotate and are refreshing on the target date are not rotated
• salts that are old enough to rotate but are not refreshing on target date are not rotated
• refresh_from dates are updated appropriately