New memroy laryout for LArray

[xerial]

Here is a brief sketch of a new LArray memory layout:

| header (8 bytes) | (raw memory contents ... ) |

Header describes a memory type (malloc, mmap, NUMA, etc.) and the subsequent array length.

New header format:

memory type (4 bits)

0000: raw memory (malloced. It can be released by Unsafe.freeMemory()) 0001: mmap (no subsequent raw memory contents. Just hold the address of the mmap) 0010: NUMA memory (needs to be deallocated by Numa.free()) 0011 - 1111: (preserved for future use)

memory address (60 bits)

With this change, we no longer need to distinguish various types of memories allocated by LArray, and would simplify the codebase.