When the chunker is not able to fill a chunk with data, it allocated a new buffer for the partial chunk of data. With many files this results in allocation of many small buffers of varying sizes, leading to heap fragmentation. This PR allocates a new buffer from the pool only if doing so would save space, otherwise it uses an partially filled (over-allocated) chunk. This makes all chunker allocation sizes be powers of 2.

Heap fragmentation is reduced at the cost of some temporary over-allocation. The advantage is that the overallocation is much shorter lived than the heap fragmentation.