Remove Redis dep, replace w/ native Elixir lib

This removes the dependency on Redis for core app functionality. Rather
than using the key/value store provided by Redis, Farside now uses a
key/val store provided by [cubdb](https://github.com/lucaong/cubdb) for
identical functionality but without reliance on a non-Elixir service.

This solution was chosen instead of ets, because storing instance data
in memory leads to a period of broken functionality whenever the app
restarts and hasn't re-populated instance data yet. It was also chosen
instead of dets, because the documentation for dets was pretty hard to
understand at first glance.

Tests and the CI build were updated to reflect the removed dependency on
Redis.

New environment variable `FARSIDE_DATA_DIR` can be used to point to a
directory where the instance data can be stored by cubdb.

Co-authored-by: Jason Clark <mithereal@gmail.com>

lib/farside.ex

@@ -35,15 +35,14 @@ defmodule Farside do
   }
 
   def get_services_map do
-    {:ok, service_list} = Redix.command(:redix, ["KEYS", "#{@service_prefix}*"])
+    service_list = CubDB.select(CubDB)
+                   |> Stream.map(fn {key, _value} -> key end)
+                   |> Stream.filter(fn key -> String.starts_with?(key, @service_prefix) end)
+                   |> Enum.to_list
 
     # Match service name to list of available instances
     Enum.reduce(service_list, %{}, fn service, acc ->
-      {:ok, instance_list} =
-        Redix.command(
-          :redix,
-          ["LRANGE", service, "0", "-1"]
-        )
+      instance_list = CubDB.get(CubDB, service)
 
       Map.put(
         acc,
@@ -58,7 +57,7 @@ defmodule Farside do
   end
 
   def get_service(service) do
-    # Check if service has an entry in Redis, otherwise try to
+    # Check if service has an entry in the db, otherwise try to
     # match against available parent services
     service_name = cond do
       !check_service(service) ->
@@ -76,42 +75,18 @@ defmodule Farside do
 
   def check_service(service) do
     # Checks to see if a specific service has instances available
-    # in redis
-    {:ok, instances} =
-      Redix.command(
-        :redix,
-        [
-          "LRANGE",
-          "#{@service_prefix}#{service}",
-          "0",
-          "-1"
-        ]
-      )
+    instances = CubDB.get(CubDB, "#{@service_prefix}#{service}")
 
-    Enum.count(instances) > 0
+    instances != nil && Enum.count(instances) > 0
   end
 
   def last_instance(service) do
     # Fetches the last selected instance for a particular service
-    {:ok, previous} =
-      Redix.command(
-        :redix,
-        ["GET", "#{service}#{@previous_suffix}"]
-      )
-    previous
+    CubDB.get(CubDB, "#{service}#{@previous_suffix}")
   end
 
   def pick_instance(service) do
-    {:ok, instances} =
-      Redix.command(
-        :redix,
-        [
-          "LRANGE",
-          "#{@service_prefix}#{service}",
-          "0",
-          "-1"
-        ]
-      )
+    instances = CubDB.get(CubDB, "#{@service_prefix}#{service}")
 
     # Either pick a random available instance,
     # or fall back to the default one
@@ -127,21 +102,12 @@ defmodule Farside do
             Enum.filter(instances, &(&1 != last_instance(service)))
             |> Enum.random()
 
-          Redix.command(
-            :redix,
-            ["SET", "#{service}#{@previous_suffix}", instance]
-          )
+          CubDB.put(CubDB, "#{service}#{@previous_suffix}", instance)
 
           instance
         end
       else
-        {:ok, result} =
-          Redix.command(
-            :redix,
-            ["GET", "#{service}#{@fallback_suffix}"]
-          )
-
-        result
+        CubDB.get(CubDB, "#{service}#{@fallback_suffix}")
       end
     instance
   end
@@ -165,12 +131,6 @@ defmodule Farside do
   end
 
   def get_last_updated do
-    {:ok, last_updated} =
-      Redix.command(
-        :redix,
-        ["GET", "last_updated"]
-      )
-
-    last_updated
+    CubDB.get(CubDB, "last_updated")
   end
 end