Off Heap Size Estimation rounds improperly

UnpooledOffHeapMemoryAllocator.estimateSizeOverhead was supposed to round up to the nearest 16. Unfortunately it is rounding improperly. Instead of adding 15 and modding by 15 it is instead adding 15 and modding by hex 15 (or 21 in base 10). This causes it to not work in many cases.

https://github.com/infinispan/infinispan/blob/master/core/src/main/java/org/infinispan/container/offheap/UnpooledOffHeapMemoryAllocator.java#L75