Recycle actor heap chunks after GC instead of returning to pool

[dipinhora]

Before this commit, any unused chunks after actor heap garbage collection would be destroyed and returned to the memory pool immediately for reuse by the runtime or any actor.

This commit changes things so that instead of destroying and returning the chunks immediatelly, we assume the actor will likely need more memory as it runs more behaviors and keep the recently unused chunks around in case that happens. This is generally more efficient than destroying a chunk and getting a new one from the memory pool because both destorying a chunk and allocating a new one involve updating the pagemap for the chunk to indicate which actor owns the chunk. Updating the pagemap is an expensive operation which we can avoid if we recycle the chunks instead. The main drawback is that since actors will no longer return chunks to the memory pool immediately after a GC, the overall system might end up using more memory as any freed chunks can only be reused by the actor that owns them and the runtime and other actors can no longer reuse that memory as they previously might have been able to.