Prevent same instance from being selected twice in a row

Introduces a new db key "<service>-previous" to track which instance was
last selected for a particular service. This allows for filtering the
list of available instances to exclude the instance that was last
picked, to ensure a (slightly) more even distribution of traffic.
There's still the possiblity of the following scenario, however:

:service instances > 2

/:service request #1 -> instance #1
/:service request #2 -> instance #2
/:service request #3 -> instance #1
/:service request #4 -> instance #2

where there are many ignored instances for a particular service. One
possible solution would be to implement the "<service>-previous" value
to be a list, rather than a single value, and push to that list until
only one element is left in the original "instance" array after
filtering, and then delete the "<service>-previous" key.

lib/farside.ex

@@ -1,6 +1,7 @@
 defmodule Farside do
   @service_prefix Application.fetch_env!(:farside, :service_prefix)
-  @fallback_str Application.fetch_env!(:farside, :fallback_str)
+  @fallback_suffix Application.fetch_env!(:farside, :fallback_suffix)
+  @previous_suffix Application.fetch_env!(:farside, :previous_suffix)
 
   def get_services_map do
     {:ok, service_list} = Redix.command(:redix, ["KEYS", "#{@service_prefix}*"])
@@ -41,12 +42,34 @@ defmodule Farside do
     # or fall back to the default one
     instance =
       if Enum.count(instances) > 0 do
-        Enum.random(instances)
+        if Enum.count(instances) == 1 do
+          # If there's only one instance, just return that one...
+          List.first(instances)
+        else
+          # ...otherwise pick a random one from the list, ensuring
+          # that the same instance is never picked twice in a row.
+          {:ok, previous} =
+            Redix.command(
+              :redix,
+              ["GET", "#{service}#{@previous_suffix}"]
+            )
+
+          instance =
+            Enum.filter(instances, &(&1 != previous))
+            |> Enum.random()
+
+          Redix.command(
+            :redix,
+            ["SET", "#{service}#{@previous_suffix}", instance]
+          )
+
+          instance
+        end
       else
         {:ok, result} =
           Redix.command(
             :redix,
-            ["GET", "#{service}#{@fallback_str}"]
+            ["GET", "#{service}#{@fallback_suffix}"]
           )
 
         result