FIX: Don't use Redis#keys in production.

As per the documentation for KEYS

```
Warning: consider KEYS as a command that should only be used in production environments with extreme care. It may ruin performance when it is executed against large databases. This command is intended for debugging and special operations, such as changing your keyspace layout.
```

Instead SCAN

```
Since these commands allow for incremental iteration, returning only a small number of elements per call, they can be used in production without the downside of commands like KEYS or SMEMBERS that may block the server for a long time (even several seconds) when called against big collections of keys or elements.
```

lib/cache.rb

@@ -22,7 +22,7 @@ class Cache < ActiveSupport::Cache::Store
   end
 
   def keys(pattern = "*")
-    redis.keys("#{@namespace}:#{pattern}")
+    redis.scan_each(match: "#{@namespace}:#{pattern}").to_a
   end
 
   def clear

lib/discourse_redis.rb

@@ -201,6 +201,31 @@ class DiscourseRedis
     end
   end
 
+  def scan_each(options = {}, &block)
+    DiscourseRedis.ignore_readonly do
+      match = options[:match].presence || '*'
+
+      options[:match] =
+        if @namespace
+          "#{namespace}:#{match}"
+        else
+          match
+        end
+
+      if block
+        @redis.scan_each(options) do |key|
+          key = remove_namespace(key) if @namespace
+          block.call(key)
+        end
+      else
+        @redis.scan_each(options).map do |key|
+          key = remove_namespace(key) if @namespace
+          key
+        end
+      end
+    end
+  end
+
   def keys(pattern = nil)
     DiscourseRedis.ignore_readonly do
       pattern = pattern || '*'
@@ -253,4 +278,10 @@ class DiscourseRedis
     Cache.new
   end
 
+  private
+
+  def remove_namespace(key)
+    key[(namespace.length + 1)..-1]
+  end
+
 end

spec/components/discourse_redis_spec.rb

@@ -35,15 +35,22 @@ describe DiscourseRedis do
 
         expect(redis.keys).to include('key')
         expect(redis.keys).to_not include('key2')
+        expect(redis.scan_each.to_a).to eq(['key'])
+
+        redis.scan_each.each do |key|
+          expect(key).to eq('key')
+        end
 
         redis.del('key')
 
         expect(raw_redis.get('default:key')).to eq(nil)
+        expect(redis.scan_each.to_a).to eq([])
 
         raw_redis.set('default:key1', '1')
         raw_redis.set('default:key2', '2')
 
         expect(redis.mget('key1', 'key2')).to eq(['1', '2'])
+        expect(redis.scan_each.to_a).to contain_exactly('key1', 'key2')
       end
     end