Add a counter to track the batches too large for the pre-allocated memory pool

[fdartayre]

#493 prints a log line when allocations are done outside of the pre-allocated memory pool. It helps users to set the right maxOrder value and avoid slow allocation.

This log is not printed for every batch above the threshold. It's only printed when the required maxOrder is bigger than the previously logged ones. It would be useful to also increment a counter to track how many batches were allocated outside the pool.

cc @andsel