Add options to re-enable the caching but disable it by default

An embarassingly long time ago it was pointed out that the MET FEX's caching of results can have unexpected and undesirable outcomes. Therefore we want to disable this behaviour.

In order to allow re-introducing it if we really want to I kept the old behaviour hidden behind a boolean guard.

This should hopefully solve ATR-19777.

I'm removing the WIP mark as I now think I understand the changes this introduces. Unfortunately they were a little more wide-ranging than expected which suggests that more things were affected by the bug than we previously believed.