Define right abstraction for VALUE EVICTION vs FULL EVICTION policies

All the front end op handling functions like VBucket::deleteItem(), VBucket::setWithMeta(), VBucket::set() etc.. are pretty long and there are myriad of if-else conditions which decide the outcome.
We should rather have a right abstraction on how the eviction policies have an impact on the front end ops. And the impact of these should be got by calling a function rather than a myriad of if-else conditions.