concurrent collection gc

I am wandering whether deferred_free is capable to handle concurrent GC.

My idea was to mimic epoch gc but
instead of checking the garbage pile on each access, we can defer the free operation to mimalloc and let it free the garbage removed two epochs away.

Ideally it may improve the performance but I am not sure whether it will enlarge the memory pressure.

Start by locating the existing deferred_free implementation and any epoch GC handling in mimalloc. Assess whether deferring frees across two epochs is safe for concurrent GC, then measure performance and memory pressure to determine whether the proposed approach is viable.

Written by the indexing model from the issue text.