option to preallocate in contiguous memory in cc_pool

[kevyang]

We should allow the option to force contiguous memory for preallocated pools of objects. This may be useful for resource pools where we want memory locality.

This most likely already happens because preallocation calls the allocate function in a tight loop. However, we rely on the memory allocator for this behavior, and depending on the implementation of it we may or may not actually end up with contiguous memory for a preallocated pool.