Remove custom pool chunk size for StoreEntry.

Remove custom pool chunk size for StoreEntry.

As an historic optimization StoreEntry uses a custom pool chunk size of 2MB.
Knowledge of the actual benefits from this optimization has been lost in time,
and it's not possible to accurately measure its actual impact in all load
scenarios; at the same time this optimization is blocking other potentially
useful developments.
This change is therefore considered a potential performance regression in
some load scenarios.