reverseproxy: Improve hostByHashing distribution (#5229)

* If upstreams are all using same host but with different ports
ie:
foobar:4001
foobar:4002
foobar:4003
...
Because fnv-1a has not a good enough avalanche effect
Then the hostByHashing result is not well balanced over
all upstreams

As last byte FNV input tend to affect few bits, the idea is to change
the concatenation order between the key and the upstream strings
So the upstream last byte have more impact on hash diffusion

modules/caddyhttp/reverseproxy/selectionpolicies.go

@@ -528,7 +528,7 @@ func hostByHashing(pool []*Upstream, s string) *Upstream {
 		if !up.Available() {
 			continue
 		}
-		h := hash(s + up.String()) // important to hash key and server together
+		h := hash(up.String() + s) // important to hash key and server together
 		if h > highestHash {
 			highestHash = h
 			upstream = up