[Sheepdog] add some candy helpers

[Liu Yuan]

These candy helpers are spin-offs from the development "Farm". I don't know
when Farm will be released, so I think it would be useful for other dev to include
these helerps in early. Most are taken from Linux kernel, git.

V3 changes:
 - move util.c in lib/, then logger and collie can call these funcs.

v2 changes:
 - drop simple slab allocator
 - drop hash_long(), use fnv_64a_buf() as kazum suggested.
 - add xpread, xpwrite helpers.

Thanks,
Yuan