Investigate possible concurrency issues in mempool

## Brief feature description

Some todos in `mem_pool.cpp` indicate that certain lockfree constructs needs to be introduced for the mempool. Furthermore, we have certain requirements for the used chunk lists were it is maybe unclear if the list actually fulfills them.

Start by reading mem_pool.cpp and the TODOs concerning lock-free constructs and chunk-list requirements. Trace the mempool's concurrency assumptions and determine whether the current chunk lists satisfy those requirements; done means documenting the findings and defining the necessary scope for a safe implementation.

Written by the indexing model from the issue text.