classes promises should not be reevaluated needlessly (persistent and non-persistent)

Requested by customer.

Take this example:

@classes: "x" expression => returnszero("COMMAND");@

when @x@ is evaluated and set, there is no need to call @COMMAND@ again, yet we call it at least twice per agent run (regardless of whether @x@ is persistent or not). This can be avoided with just

@classes: !x:: "x" expression => returnszero("COMMAND");@

but it's tedious to guard every class. I propose to add this guard automatically to every classes promise that calls a system function, at least when @cache_system_functions@ is set.

This should work for persistent classes too.

Target version and priority TBD.