bugfix: MemoryContextAllocHuge() instead of palloc() for large chunk

multihash_preload_khashtable may allocate more than 1GB chunk if
zone-length and table datasize are large, however, palloc()
internally has hard limitation by 1GB.
So, we had to use MemoryContextAllocHuge() to allocate more than
the boundary.
Merge branch 'master' into numeric