Allow more than 2 checkpoints

EPEngine already allows any max number of checkpoints in the CM::CheckpointList:

        "max_checkpoints": {

            "default": "2",

            "dynamic": true,

            "type": "size_t"

        },

The problem with increasing that number is that we have O(N = ckpt-list-size) code around that executes under CM::lock, so that possibly blocking frontend operations and degrading (massively, depending on the CheckpointList size) frontend throughput.

So essentially for allowing a higher number of checkpoints we need to remove any O(N = ckpt-list-size) code, which is what this MB tracks.
The most (in)famous example is the code in CheckpointManager::removeClosedUnrefCheckpoints, where we acquire the CM::lock and scan the entire CheckpointList.

At the time of writing the main purpose of this work is helping in the memory deallocation of checkpoints. Eg see MB-43299.

Also, this is a pre-requirement for MB-47388 (fine-grained locking in CM).