Add Caching to CBA_fnc_compatibleItems

[jokoho48]

adding Caching for config values based on #260

[PabstMirror]

There never is a call to setVariable to save the results in the cache.
The filter only runs on a cache hit, so would be ignored on the first call.
There is no proper return on a cache miss, it always just returned 0 from the last count statement.
There is an extra private ["_compatibleItems"]; inside the if (isNil "_compatibleItems") then { so there would be scope issues on that variable.

@jokoho48 look at d69a097 for fixes and a test case.
Can probably just PR that branch if you are ok with it.