Move code specific to the classic plan cache out of plan_cache.h

We will have three separate plan cache related files:

• plan_cache.h will house the common code shared between the classic and SBE plan caches.
• sbe_plan_cache.h for plan cache code specific to the SBE plan cache.
• classic_plan_cache.h for plan cache code specific to the classic plan cache.

The goal of this ticket is to create classic_plan_cache.h, by moving any code which belongs there out of plan_cache.h.