This reverts commit b585addc23de25c019f250986d50c0373219571d.
The allocation of MemoryPool's page memory is doing in MemoryPool::allocPage,
which uses ::malloc, thus we need to use ::free to free the page memory in
MemoryPool::freeUnusedPages.
Memory is allocated with new in Hashmap::allocNode() and incorrectly
freed with free() in ~MemoryPool() and freeUnusedPages().
Issue reported by Dr. Memory