MINOR: backend: support hash-key guid for a stabler distribution

MINOR: backend: support hash-key guid for a stabler distribution

When server fleets are constantly updated, using a stable distribution
across a bunch of load balancers can be convenient. The addr and port
already provide a bit of this but for situations were addresses might
differ between sites or change dynamically this does not work. The guid
is perfect for this because by definition it's supposed to designate a
single server and be unique. So when two servers anywhere have the same,
the tool that provisionned them promises that they are the same server.

So here we introduce "hash-key guid" which performs a 32-bit hash on
the GUID value. When no guid is provided, a fallback is performed on
ID, as is done for other keys.